Add infrastructure for loading icon themes from extensions (#23203)

This PR adds the supporting infrastructure to support loading icon
themes defined by extensions.

Here's an example icon theme:

```json
{
  "name": "My Icon Theme",
  "author": "Me <me@example.com>",
  "themes": [
    {
      "name": "My Icon Theme",
      "appearance": "dark",
      "file_icons": {
        "gleam": { "path": "./icons/file_type_gleam.svg" },
        "toml": { "path": "./icons/file_type_toml.svg" }
      }
    }
  ]
}
```

The icon paths are resolved relative to the root of the extension
directory.

Release Notes:

- N/A

crates/extension/src/extension_manifest.rs

@@ -70,6 +70,8 @@ pub struct ExtensionManifest {
     #[serde(default)]
     pub themes: Vec<PathBuf>,
     #[serde(default)]
+    pub icon_themes: Vec<PathBuf>,
+    #[serde(default)]
     pub languages: Vec<PathBuf>,
     #[serde(default)]
     pub grammars: BTreeMap<Arc<str>, GrammarManifestEntry>,
@@ -199,6 +201,7 @@ fn manifest_from_old_manifest(
             themes.dedup();
             themes
         },
+        icon_themes: Vec::new(),
         languages: {
             let mut languages = manifest_json.languages.into_values().collect::<Vec<_>>();
             languages.sort();