fix: simplify `ImageField`

[angeloashmore]

Types of changes

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

Description

This PR simplifies ImageField by changing how it uses handles ThumbnailNames when given null as its value.

Deprecating null in ThumbnailNames

As of this PR, ImageField<never> is the preferred method to type an Image field without thumbnails. ImageField<null> will continue to work, but null is now depreacted.

Reason for the change

@prismicio/helpers's use of ImageField in its isFilled.image() helper resulted in a broken index.d.ts file in @prismicio/helpers. The compiled type declaration file, bundled by rollup-plugin-dts, included invalid types.

See the following issues for more details:

• prismicio/prismic-client#252
• prismicio/prismic-helpers#55

The simplification made in this PR removes the invalid generated code in @prismicio/helpers and @prismicio/client.

Checklist:

• [ ] 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-commenter]

Codecov Report

Merging #46 (930a668) into master (5b4d5e5) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #46   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines         1768      1766    -2     
  Branches         7         7           
=========================================
- Hits          1768      1766    -2     

Impacted Files	Coverage Δ
src/fields.ts	100.00% <100.00%> (ø)

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