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>

Cargo.lock

@@ -5338,7 +5338,6 @@ dependencies = [
  "tree-sitter-c",
  "tree-sitter-cpp",
  "tree-sitter-css",
- "tree-sitter-dart",
  "tree-sitter-elixir",
  "tree-sitter-elm",
  "tree-sitter-embedded-template",
@@ -10217,15 +10216,6 @@ dependencies = [
  "tree-sitter",
 ]
 
-[[package]]
-name = "tree-sitter-dart"
-version = "0.0.1"
-source = "git+https://github.com/agent3bood/tree-sitter-dart?rev=48934e3bf757a9b78f17bdfaa3e2b4284656fdc7#48934e3bf757a9b78f17bdfaa3e2b4284656fdc7"
-dependencies = [
- "cc",
- "tree-sitter",
-]
-
 [[package]]
 name = "tree-sitter-elixir"
 version = "0.1.0"
@@ -12490,6 +12480,13 @@ dependencies = [
  "zed_extension_api 0.0.4",
 ]
 
+[[package]]
+name = "zed_dart"
+version = "0.0.1"
+dependencies = [
+ "zed_extension_api 0.0.6",
+]
+
 [[package]]
 name = "zed_emmet"
 version = "0.0.1"
@@ -12526,6 +12523,8 @@ dependencies = [
 name = "zed_extension_api"
 version = "0.0.6"
 dependencies = [
+ "serde",
+ "serde_json",
  "wit-bindgen",
 ]
 
@@ -12575,7 +12574,7 @@ dependencies = [
 name = "zed_svelte"
 version = "0.0.1"
 dependencies = [
- "zed_extension_api 0.0.4",
+ "zed_extension_api 0.0.6",
 ]
 
 [[package]]