Add `feature/tree-sitter` derivation to emacs overlay

[yilinwei]

Fixes #226 for Linux.

I don't have a Mac to get this to work and test it for Darwin. I think it will be a matter of switching .so to the required extension.

Note, the dynamic libraries need to be set during the buildPhase otherwise the build fails. As more languages get added into emacs core, more tree-sitter-grammars need to be added.

[aciceri]

This is without native compilation, right? Would be possible to have also a version with native compilation?

[jian-lin]

This is without native compilation, right?

This is with native compilation like all emacs variants in this overlay and nixpkgs.

[aciceri]

This is with native compilation like all emacs variants in this overlay and nixpkgs.

Oh, it's been a while since I last used this overlay... By the way maybe we want to make Hydra build this emacsen too?

[bhankas]

With 29.0 branch-off expected in less than 2 months, I think we can simply wait and make tree-sitter default for unstable builds, as it appears will be the case upstream.

[yilinwei]

/cc @leungbk @jian-lin

Updated PR based on comments.

[leungbk]

Thanks!