Add logic for managing language and theme extensions (#7467)

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>

crates/zed/Cargo.toml

@@ -44,6 +44,7 @@ db.workspace = true
 diagnostics.workspace = true
 editor.workspace = true
 env_logger.workspace = true
+extension.workspace = true
 feature_flags.workspace = true
 feedback.workspace = true
 file_finder.workspace = true