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/project/src/project_settings.rs

@@ -47,7 +47,7 @@ pub struct BinarySettings {
     pub arguments: Option<Vec<String>>,
 }
 
-#[derive(Clone, Debug, Serialize, Deserialize, PartialEq, Eq, JsonSchema)]
+#[derive(Clone, Debug, Default, Serialize, Deserialize, PartialEq, Eq, JsonSchema)]
 #[serde(rename_all = "snake_case")]
 pub struct LspSettings {
     pub binary: Option<BinarySettings>,