feat: add `@prismicio/client/richtext` entry

Types of changes

[ ] Chore (a non-breaking change which is related to package maintenance)
[ ] Bug fix (a non-breaking change which fixes an issue)
[x] New feature (a non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

This PR adds a new @prismicio/client/richtext entry.

It ports all of @prismicio/richtext to @prismicio/client as an isolated entry. An isolated entry was selected over exporting it adjacent to all other helpers to minimize accidental usage; rich text helpers are low-level functions to be used primarily in high-level framework SDKs, like @prismicio/react and @prismicio/vue.

@prismicio/richtext will be deprecated once this PR is published. All packages dependent on @prismicio/richtext should be upgraded to @prismicio/client/richtext.

For more details behind this PR's motivation, see this thread: https://github.com/prismicio/prismic-richtext/pull/63#discussion_r1296975156

Migration guide

Update to the latest version of @prismicio/client:
```
npm install @prismicio/client@latest
```

Replace all @prismicio/richtext imports with @prismicio/client/richtext:

- import { serialize } from "@prismicio/richtext";
+ import { serialize } from "@prismicio/client/richtext";

Uninstall @prismicio/richtext:
```
npm uninstall @prismicio/richtext
```

Checklist:

[x] My change requires an update to the official documentation.
[x] All TSDoc comments are up-to-date and new ones have been added where necessary.
[x] All new and existing tests are passing.

🦕

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (b43557d) 99.96% compared to head (f63f65e) 99.97%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #318 +/- ## ======================================== Coverage 99.96% 99.97% ======================================== Files 50 57 +7 Lines 6251 6794 +543 Branches 332 374 +42 ======================================== + Hits 6249 6792 +543 Misses 2 2 ``` | [Files](https://app.codecov.io/gh/prismicio/prismic-client/pull/318?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio) | Coverage Δ | | |---|---|---| | [src/helpers/asHTML.ts](https://app.codecov.io/gh/prismicio/prismic-client/pull/318?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio#diff-c3JjL2hlbHBlcnMvYXNIVE1MLnRz) | `100.00% <100.00%> (ø)` | | | [src/helpers/asText.ts](https://app.codecov.io/gh/prismicio/prismic-client/pull/318?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio#diff-c3JjL2hlbHBlcnMvYXNUZXh0LnRz) | `100.00% <100.00%> (ø)` | | | [src/index.ts](https://app.codecov.io/gh/prismicio/prismic-client/pull/318?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio#diff-c3JjL2luZGV4LnRz) | `100.00% <100.00%> (ø)` | | | [src/lib/serializerHelpers.ts](https://app.codecov.io/gh/prismicio/prismic-client/pull/318?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio#diff-c3JjL2xpYi9zZXJpYWxpemVySGVscGVycy50cw==) | `100.00% <100.00%> (ø)` | | | [src/richtext/asText.ts](https://app.codecov.io/gh/prismicio/prismic-client/pull/318?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio#diff-c3JjL3JpY2h0ZXh0L2FzVGV4dC50cw==) | `100.00% <100.00%> (ø)` | | | [src/richtext/asTree.ts](https://app.codecov.io/gh/prismicio/prismic-client/pull/318?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio#diff-c3JjL3JpY2h0ZXh0L2FzVHJlZS50cw==) | `100.00% <100.00%> (ø)` | | | [src/richtext/composeSerializers.ts](https://app.codecov.io/gh/prismicio/prismic-client/pull/318?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio#diff-c3JjL3JpY2h0ZXh0L2NvbXBvc2VTZXJpYWxpemVycy50cw==) | `100.00% <100.00%> (ø)` | | | [src/richtext/index.ts](https://app.codecov.io/gh/prismicio/prismic-client/pull/318?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio#diff-c3JjL3JpY2h0ZXh0L2luZGV4LnRz) | `100.00% <100.00%> (ø)` | | | [src/richtext/serialize.ts](https://app.codecov.io/gh/prismicio/prismic-client/pull/318?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio#diff-c3JjL3JpY2h0ZXh0L3NlcmlhbGl6ZS50cw==) | `100.00% <100.00%> (ø)` | | | [src/richtext/types.ts](https://app.codecov.io/gh/prismicio/prismic-client/pull/318?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio#diff-c3JjL3JpY2h0ZXh0L3R5cGVzLnRz) | `100.00% <100.00%> (ø)` | | | ... and [1 more](https://app.codecov.io/gh/prismicio/prismic-client/pull/318?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio) | |

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prismicio / prismic-client