This PR enables `getSemanticDiagnostics()` to be called on external templates.
Several changes are needed to land this feature:
1. The adapter needs to implement two additional methods:
a. `readResource()`
Load the template from snapshot instead of reading from disk
b. `getModifiedResourceFiles()`
Inform the compiler that external templates have changed so that the
loader could invalidate its internal cache.
2. Create `ScriptInfo` for external templates in MockHost.
Prior to this, MockHost only track changes in TypeScript files. Now it
needs to create `ScriptInfo` for external templates as well.
For (1), in order to make sure we don't reload the template if it hasn't
changed, we need to keep track of its version. Since the complexity has
increased, the adapter is refactored into its own class.
PR Close#39065
Keen Yee Liau