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/zed/Cargo.toml

@@ -87,6 +87,7 @@ serde_json.workspace = true
 settings.workspace = true
 simplelog = "0.9"
 smol.workspace = true
+snippet_provider.workspace = true
 tab_switcher.workspace = true
 supermaven.workspace = true
 task.workspace = true