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/snippet_provider/Cargo.toml

@@ -14,6 +14,7 @@ collections.workspace = true
 fs.workspace = true
 futures.workspace = true
 gpui.workspace = true
+parking_lot.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 snippet.workspace = true