This PR adds the initial support for loading extensions in Zed.
### Extensions Directory
Extensions are loaded from the extensions directory.
The extensions directory has the following structure:
```
extensions/
installed/
extension-a/
grammars/
languages/
extension-b/
themes/
manifest.json
```
The `manifest.json` file is used internally by Zed to keep track of
which extensions are installed. This file should be maintained
automatically, and shouldn't require any direct interaction with it.
Extensions can provide Tree-sitter grammars, languages, and themes.
Release Notes:
- N/A
---------
Co-authored-by: Marshall <marshall@zed.dev>
This opens up a possibility of supporting multiple comment continuation
flavours in editor, e.g. doc comments for Rust (which we seize as well
in this commit). Only the first `line_comment` value is used for
Editor::ToggleComments
Fixes: https://github.com/zed-industries/zed/issues/6692
Release Notes:
- Added support for doc-comment continuations in Rust language.
Max BrunsfeldandMarshall
Piotr Osiewicz
Joseph T. Lyons
Nate Butler
KCaverly
Kay Simmons
