feat(help): generate :helptags using Treesitter

Problem:
Tags are manually parsed in C which is not flexible and prone to errors.
Extending the help system to allow for other formats (e.g. Markdown)
would require a large rewrite in the C core, while with Treesitter it
only needs a query update.

Solution:
Use the power of treesitter to extract the tags from helpfiles.

- build: set `$VIMRUNTIME` when generating helptags, like
  `cmake/Util.cmake` already does for other generators.
- fix(help): only accept tags delimited by whitespace. The old C parser
  only accepted a `*tag*` preceded by start-of-line or whitespace and
  followed by whitespace or end-of-line. The vimdoc parser also captures
  tags followed by other text, e.g. `*$XDG_STATE_HOME*/.../logs` in
  starting.txt, which caused an E154 duplicate tag error for docs that
  were previously fine.

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