Add hash-pipe semantic highlighting, make validation live

[vezwork]

Fixes #409

• Adds semantic highlighting for hash-pipe comments in code blocks
• Yaml validation already existed, but makes it so yaml validation (in both frontmatter and in hash-pipe comments) happens as the user types rather than on-save.

Design considerations

• According to an LLM, the yaml frontmatter highlighting in qmd files is done by "declarative grammar delegation". This approach is ideal because it delegates highlighting of the yaml in frontmatters to the yaml textmate grammar in VSCode (so its consistent with how yaml would be highlight in a .yaml file). I would've liked to use this approach for hash-pipe comments, but it does not sound like it possible, since the hash-pipe comments break up the yaml and it sounds like delegation requires a contiguous block of yaml.

TODO

• test in VSCode
• test in Positron

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