Introduce storage highlighting for typescript/javascript

[jakenvac]

Inspired by and following the patterns of #2731.
Adds keyword.storage.type and keyword.storage.modifier scopes for syntax highlighting.

[the-mikedavis]

Could you add some docs on the new keys?

helix/book/src/themes.md

Line 154 in e58d28a

- `storage` - Keywords that affect the storage of a variable, function or data structure `static`, `mut`, `const`, `ref`

[jakenvac]

Could you add some docs on the new keys?

All done. Feedback on the changes are welcome. I've slightly moved around the existing storage description as it was more specific to the modifier sub-scope.

I've added example keywords in rust as that was what was already there, however my personal opinion is that we shouldn't provide language specific examples. For a start I think those keywords are probably not implemented in the rust highlighting query.

I'm also not a rust dev so please let me know if I've misunderstood some of the keywords and they don't fall into the categories I've described.

[the-mikedavis]

keyword.storage was originally based on Vim's StorageClass highlight group (#2731). I think textmate's use of storage.{type,modifier} is a bit clearer though so let's lean towards those. For consistency though, Rust's const should become keyword.storage.type.

[the-mikedavis]

Looks good. Thanks!