This commit adds basic support for tree-sitter in the emacs build,
such that (if the user opts into tree-sitter support), tree-sitter
will be enabled and binary library files for tree-sitter can be
included in the `lib` directory of packages passed to
`emacsWithPackages`. The libraries will be aggregated and included in
treesit-extra-load-path.
The previous pattern for this in the community was to add tree-sitter
libaries by patching emacs's `RUNPATH` with `patchelf` in a post-fixup
phase. However, this has the substantial drawback that two different
emacs installations with different lists of available tree-sitter
libraries must be entirely separate builds. By supplying the
tree-sitter libraries in the wrapping layer of `emacsWithpackages`, it
becomes possible to share a single, more-cacheable "core emacs".
This support defaults to "on" only in emacs 29 and up, since previous
versions do not support tree-sitter out of the box.
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