Added and refactored exports of the schema-blocks package

[hansjovis]

Summary

This PR can be summarized in the following changelog entry:

• [schema-blocks] Moves the initialize function to its own file, to clean up the index.ts file of the package.

Relevant technical choices:

• Moved the initialize function to its own file, to clean up the index.ts file of the package.
  • Its only function is to import and expose functionality, not to implement it.
• This is needed for the Apply button to work.

Test instructions

This PR can be tested by following these steps:

• Companion PR of https://github.com/Yoast/wordpress-seo-premium/pull/3268. Please follow its test instructions.
  • Note: This PR only needs to be tested in Premium.

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 #

[andizer]

CR

[johannadevos]

Acceptance: ✅

• The acceptance test of https://github.com/Yoast/wordpress-seo-premium/pull/3268 has passed.