feat(Runtime): Add a way to display internal-only errors/messages.

This will allow, for example, pointing to documentation on GitHub externally and G3Docs (internally) without making the experience worse for one set of users - copying how we currently do it within the "angular_compiler" package.

PiperOrigin-RevId: 259822455

angular/lib/src/runtime.dart

@@ -5,6 +5,8 @@
 /// rely on.
 library angular.src.runtime;
 
+import 'runtime/messages/messages.dart';
+
 export 'runtime/check_binding.dart'
     show
         UnstableExpressionError,
@@ -14,3 +16,9 @@ export 'runtime/check_binding.dart'
         debugThrowIfChanged;
 
 export 'runtime/optimizations.dart' show isDevMode, unsafeCast;
+
+/// Formatting service for displaying runtime text or error message bodies.
+///
+/// Internally, this may be implemented differently in order to display internal
+/// only links, and likewise when exported to GitHub/Pub.
+const runtimeMessages = Messages();

angular/lib/src/runtime/messages/messages.dart

@@ -0,0 +1,4 @@
+/// Base implementation of runtime message/error formatting.
+class Messages {
+  const Messages();
+}