dart: Respect LSP binary settings (#17494)

Enable configuring Dart's LSP from other means of installation types.

Some users don't install the `dart` binary, but uses version manager.

In the example, I uses [FVM](https://fvm.app/) (short for "Flutter
Version Manager").

I have tested this with "Install Dev Extensions".

Release Notes:

- N/A

cc other maintainer: @agent3bood @flo80

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

docs/src/languages/dart.md

@@ -5,6 +5,23 @@ Dart support is available through the [Dart extension](https://github.com/zed-in
 - Tree Sitter: [UserNobody14/tree-sitter-dart](https://github.com/UserNobody14/tree-sitter-dart)
 - Language Server: [dart language-server](https://github.com/dart-lang/sdk)
 
+## Configuration
+
+The `dart` binary can be configured in a Zed settings file with:
+
+```json
+{
+  "lsp": {
+    "dart": {
+      "binary": {
+        "path": "/opt/homebrew/bin/fvm",
+        "arguments": ["dart", "language-server", "--protocol=lsp"]
+      }
+    }
+  }
+}
+```
+
 <!--
 TBD: Document Dart. pubspec.yaml
 - https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server/tool/lsp_spec/README.md