Add language_server_workspace_configuration to extension API (#10212)

This PR adds the ability for extensions to implement
`language_server_workspace_configuration` to provide workspace
configuration to the language server.

We've used the Dart extension as a motivating example for this, pulling
it out into an extension in the process.

Release Notes:

- Removed built-in support for Dart, in favor of making it available as
an extension. The Dart extension will be suggested for download when you
open a `.dart` file.

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/extension_api/Cargo.toml

@@ -15,6 +15,8 @@ workspace = true
 path = "src/extension_api.rs"
 
 [dependencies]
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
 wit-bindgen = "0.22"
 
 [package.metadata.component]