Extensions registering tasks (#9572)

This PR also introduces built-in tasks for Rust and Elixir. Note that
this is not a precedent for future PRs to include tasks for more
languages; we simply want to find the rough edges with tasks & language
integrations before proceeding to task contexts provided by extensions.

As is, we'll load tasks for all loaded languages, so in order to get
Elixir tasks, you have to open an Elixir buffer first. I think it sort
of makes sense (though it's not ideal), as in the future where
extensions do provide their own tasks.json, we'd like to limit the # of
tasks surfaced to the user to make them as relevant to the project at
hand as possible.

Release Notes:

- Added built-in tasks for Rust and Elixir files.

crates/extension/Cargo.toml

@@ -44,6 +44,8 @@ wasmtime.workspace = true
 wasmtime-wasi.workspace = true
 wasmparser.workspace = true
 wit-component.workspace = true
+task.workspace = true
+serde_json_lenient.workspace = true
 
 [dev-dependencies]
 ctor.workspace = true