dart: Add Flutter runnables and tasks (#13686)

Release Notes:

- Added runnable tasks for Flutter

extensions/dart/README.md

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+## Roadmap
+
+1. Add `dart run` command.
+2. Add `dart test` command.
+3. Add `flutter test --name` command, to allow running a single test or group of tests.
+4. Auto hot reload Flutter app when files change.

extensions/dart/languages/dart/runnables.scm

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+; Flutter main
+(
+    (
+        (import_or_export
+            (library_import
+                (import_specification
+                    ("import"
+                        (configurable_uri
+                            (uri
+                                (string_literal) @_import
+                                (#match? @_import "package:flutter/(material|widgets|cupertino).dart")
+                                (#not-match? @_import "package:flutter_test/flutter_test.dart")
+                                (#not-match? @_import "package:test/test.dart")
+        ))))))
+        (
+            (function_signature
+                name: (_) @run
+            )
+            (#eq? @run "main")
+        )
+        (#set! tag flutter-main)
+    )
+)
+
+; Flutter test main
+(
+    (
+        (import_or_export
+            (library_import
+                (import_specification
+                    ("import"
+                        (configurable_uri
+                            (uri
+                                (string_literal) @_import
+                                (#match? @_import "package:flutter_test/flutter_test.dart")
+        ))))))
+        (
+            (function_signature
+                name: (_) @run
+            )
+            (#eq? @run "main")
+        )
+        (#set! tag flutter-test-main)
+    )
+)

extensions/dart/languages/dart/tasks.json

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+[
+  {
+    "label": "flutter run",
+    "command": "flutter",
+    "args": ["run"],
+    "tags": ["flutter-main"]
+  },
+  {
+    "label": "fvm flutter run",
+    "command": "fvm flutter",
+    "args": ["run"],
+    "tags": ["flutter-main"]
+  },
+  {
+    "label": "flutter test $ZED_STEM",
+    "command": "flutter",
+    "args": ["test", "$ZED_FILE"],
+    "tags": ["flutter-test-main"]
+  },
+  {
+    "label": "fvm flutter test $ZED_STEM",
+    "command": "fvm flutter",
+    "args": ["test", "$ZED_FILE"],
+    "tags": ["flutter-test-main"]
+  }
+]