Build: Restore block parser in Core.

[dmsnell]

Quick summary

• Core no longer receives updates to the block parser from Gutenberg.
• Gutenberg will remove the PHP block parser code in the default parser.
• Nothing is supposed to change apart from Core working where it was previously broken due to the removed parser files.

Summary

Trac ticket: Core-64521
See also: Core-64393

The work in #10638 [61438] for Core-64393 removed the block parser classes from Core, which caused numerous scripts to fail because they were missing. Conditional checks were added in #10718 [61492] which left WordPress in an inoperable state.

This patch restores the block parser in Core, in preparation for work to remove it from Gutenberg (in a separate patch).

Ironically, the files were removed because the new build was copying them over from Gutenberg and the intent was to avoid having two sources of truth, but this was previously the existing mechanism, so having done nothing to the parser files would have left the status quo. This patch removes the problems originally created by removing the files. They will not be copied from Gutenberg any more and the only source of truth will be Core.

Until removed from Gutenberg, because of the build changes, any changes made on the Gutenberg side will be lost unless manually copied over.

Developed in: #10761
Discussed in: https://core.trac.wordpress.org/ticket/64521

Follow-up to [61438], [61492].
Props dmsnell, mcsf, mukesh27, youknowriad.

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Props dmsnell, youknowriad.

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[youknowriad]

This looks good to me :)

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 61504
GitHub commit: 6ba48db

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.