Block Hooks: Try Streaming Parser

[ockham]

Longer-term experiment, based on #5753. Currently just copying over code from that PR. More to come 😬

With the introduction of apply_block_hooks_to_content() in WP 6.6, which in 6.7 became the main entry point for Block Hooks across the board — regardless of whether applied to templates, patterns, Navigation blocks, or Post Content, it has become easier to swap out the existing Block Hooks implementation (which requires parsing the block markup into a tree, which is then traversed and re-serialized) for a streaming parser.

Block Hooks are fairly exhaustively covered by unit tests, but a large number of those tests covers lower-level functions that operate at block tree level. In order for this experiment to become viable, we will need to replace them with something that operates directly on block markup level -- see the below roadmap.

Roadmap:

• Plug inject_hooked_blocks_into_content into apply_block_hooks_to_content (to replace the existing block tree traversal based implementation with it).
• Fix failing unit tests.
• Take note of now-obsolete lower-level functions that are no longer invoked as part of the callstack of apply_block_hooks_to_content. Analyze what they're doing, and replace them with tests that cover apply_block_hooks_to_content to ensure equivalent behavior.

Trac ticket:

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[ockham]

Noting that #6760 might come in handy for this.

[github-actions]

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