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/languages/src/lib.rs

@@ -13,7 +13,6 @@ use self::{deno::DenoSettings, elixir::ElixirSettings};
 
 mod c;
 mod css;
-mod dart;
 mod deno;
 mod elixir;
 mod elm;
@@ -92,7 +91,6 @@ pub fn init(
         ("typescript", tree_sitter_typescript::language_typescript()),
         ("vue", tree_sitter_vue::language()),
         ("yaml", tree_sitter_yaml::language()),
-        ("dart", tree_sitter_dart::language()),
     ]);
 
     macro_rules! language {
@@ -312,7 +310,6 @@ pub fn init(
         vec![Arc::new(terraform::TerraformLspAdapter)]
     );
     language!("hcl", vec![]);
-    language!("dart", vec![Arc::new(dart::DartLanguageServer {})]);
 
     languages.register_secondary_lsp_adapter(
         "Astro".into(),