Improve the check for whether a project is Flutter to be more reliable

[DanTup]

This code does a basic regex to check for sdk: flutter in the pubspec.yaml to decide whether a project is a Flutter project or not:

Dart-Code/src/shared/utils/fs.ts

Lines 141 to 144 in b7c1124

	export function pubspecContentReferencesFlutterSdk(content: string): boolean {
	const regex = new RegExp("sdk\\s*:\\s*[\"']?flutter[\"']?", "i");
	return regex.test(content);
	}

It was done this way for simplicity when we had no yaml parser referenced. It can be tripped up by comments and other unrelated keys. Now we have a yaml parser, we should do this properly so it's reliable.

[pattobrien]

For extra context: I had the Flutter SDK dependency commented out in my pubspec file like below, yet tests were still being run as flutter test rather than dart test, causing undesired behavior.

dev_dependencies:
  # flutter:
  #   sdk: flutter
  lints: ^2.0.0