Added typings entry in schema-blocks package.json

[hansjovis]

Summary

This PR can be summarized in the following changelog entry:

• [schema-blocks] Added typings entry to package.json to make sure that other packages can consume the typings even with a linked schema-blocks package.

Relevant technical choices:

• The location of the typings file of the schema-blocks package (index.d.ts) changes based on whether it is unlinked/installed or linked.
  • Linked: located within dist/index.d.ts
  • Unlinked: located within the package root: index.d.ts.
• This means that with a linked monorepo, WordPress SEO Premium cannot find the typings file.
  • It assumes that it is located in index.d.ts, but with a linked monorepo it is located within dist/index.d.ts.
• This PR solves this by explicitly setting the path to the typings in the package.json file.
• I also adapted the prepublishOnly hook such that the typings property is updated when the package is published.
  • Like happens for main: dist/index.js -> main: index.js right now.

Test instructions

This PR can be tested by following these steps:

Impact check

• This PR affects the following parts of the plugin, which may require extra testing: *

UI changes

• [ ] This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Quality assurance

• [ ] I have tested this code to the best of my abilities
• [ ] I have added unittests to verify the code works as intended

Fixes #