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/editor/src/editor.rs

@@ -11767,7 +11767,7 @@ fn snippet_completions(
     let language = buffer.read(cx).language_at(buffer_position);
     let language_name = language.as_ref().map(|language| language.lsp_id());
     let snippet_store = project.snippets().read(cx);
-    let snippets = snippet_store.snippets_for(language_name);
+    let snippets = snippet_store.snippets_for(language_name, cx);
 
     if snippets.is_empty() {
         return vec![];