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/Cargo.toml

@@ -40,7 +40,6 @@ tree-sitter-bash.workspace = true
 tree-sitter-c.workspace = true
 tree-sitter-cpp.workspace = true
 tree-sitter-css.workspace = true
-tree-sitter-dart.workspace = true
 tree-sitter-elixir.workspace = true
 tree-sitter-elm.workspace = true
 tree-sitter-embedded-template.workspace = true