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/util/src/arc_cow.rs

@@ -1,5 +1,6 @@
 use std::{
     borrow::Cow,
+    cmp::Ordering,
     fmt::{self, Debug},
     hash::{Hash, Hasher},
     sync::Arc,
@@ -18,6 +19,18 @@ impl<'a, T: ?Sized + PartialEq> PartialEq for ArcCow<'a, T> {
     }
 }
 
+impl<'a, T: ?Sized + PartialOrd> PartialOrd for ArcCow<'a, T> {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        self.as_ref().partial_cmp(other.as_ref())
+    }
+}
+
+impl<'a, T: ?Sized + Ord> Ord for ArcCow<'a, T> {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.as_ref().cmp(other.as_ref())
+    }
+}
+
 impl<'a, T: ?Sized + Eq> Eq for ArcCow<'a, T> {}
 
 impl<'a, T: ?Sized + Hash> Hash for ArcCow<'a, T> {

crates/util/src/paths.rs

@@ -14,7 +14,7 @@ lazy_static::lazy_static! {
     pub static ref THEMES_DIR: PathBuf = HOME.join(".config/zed/themes");
     pub static ref LOGS_DIR: PathBuf = HOME.join("Library/Logs/Zed");
     pub static ref SUPPORT_DIR: PathBuf = HOME.join("Library/Application Support/Zed");
-    pub static ref PLUGINS_DIR: PathBuf = HOME.join("Library/Application Support/Zed/plugins");
+    pub static ref EXTENSIONS_DIR: PathBuf = HOME.join("Library/Application Support/Zed/extensions");
     pub static ref LANGUAGES_DIR: PathBuf = HOME.join("Library/Application Support/Zed/languages");
     pub static ref COPILOT_DIR: PathBuf = HOME.join("Library/Application Support/Zed/copilot");
     pub static ref DEFAULT_PRETTIER_DIR: PathBuf = HOME.join("Library/Application Support/Zed/prettier");