Add syntax-highlighting support for helix-editor

[mneumann]

Helix Editor is a quite capable Kakoune/sam-like text editor written in Rust, that has LSP and many other goodies built in.

Add syntax-highlighting support for Savi to Helix.

[mneumann]

Pony recently got supported by @mfelsche helix#5416.

Whoever want's to do that, also look at #453 (by @repr-man)

Tasks:

• [ ] Implement tree-sitter-savi (see tree-sitter-ponylang)
• [ ] Add relevant queries to helix (see helix#5416)