This adds tree-sitter grammars for both Elm and GLSL, with injections
for GLSL embedded within Elm. It also adds an `outline.scm` for Elm,
though limitations in the tree-sitter grammar meant that I wasn't able
to get it looking exactly how I'd have liked.

In particular, it wasn't clear how to nicely annotate functions in the
outline as being functions, or how to prevent the fields of a record
declaration from being increasingly indented.


![image](https://github.com/zed-industries/zed/assets/285821/544bc00b-3bfc-4ec7-be9d-764b9f0292ab)

![image](https://github.com/zed-industries/zed/assets/285821/74e57e95-bf87-4989-ae29-a2f625141bcf)

![image](https://github.com/zed-industries/zed/assets/285821/9e283c78-66d5-4c15-9827-1b5b446cdc37)

fixes https://github.com/zed-industries/community/issues/598

Release Notes:
- Added syntax highlighting for the Elm and GLSL languages