CSS data source (#23)

With this we should have what we need for completions and hover
information down the road.

pkgs/sass_language_services/analysis_options.yaml

@@ -1,3 +1,3 @@
 include: package:sass_analysis/analysis_options.yaml
 analyzer:
-  exclude: ["**/*.pb*.dart"]
+  exclude: ["**/*.pb*.dart", "**/*_generated.dart"]

pkgs/sass_language_services/lib/src/css/css_at_directive.dart

@@ -0,0 +1,13 @@
+import 'entry_status.dart';
+import 'reference.dart';
+
+class CssAtDirective {
+  String name;
+  String? description;
+  List<String>? browsers;
+  EntryStatus? status;
+  List<Reference>? references;
+
+  CssAtDirective(this.name,
+      {this.description, this.browsers, this.status, this.references});
+}

pkgs/sass_language_services/lib/src/css/css_color_function.dart

@@ -0,0 +1,9 @@
+class CssColorFunction {
+  String label;
+  String function;
+  String insertSnippet;
+  String description;
+
+  CssColorFunction(
+      this.label, this.function, this.insertSnippet, this.description);
+}