extensions: Add support for snippets provided by extensions (#14020)

For now extensions can only register global snippets, but there'll be
follow-up work to support scope attribute in snippets.json.

Release Notes:

- Extensions can now provide snippets by including `snippets.json` file
next to the extension manifest.

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/extension/src/extension_builder.rs

@@ -526,6 +526,11 @@ fn populate_defaults(manifest: &mut ExtensionManifest, extension_path: &Path) ->
         }
     }
 
+    let snippets_json_path = extension_path.join("snippets.json");
+    if snippets_json_path.exists() {
+        manifest.snippets = Some(snippets_json_path);
+    }
+
     // For legacy extensions on the v0 schema (aka, using `extension.json`), we want to populate the grammars in
     // the manifest using the contents of the `grammars` directory.
     if manifest.schema_version.is_v0() {