This is based on a discussion we had a few weeks ago. Currently if a component uses `ViewEncapsulation.ShadowDom` and its selector doesn't meet the requirements for a custom element tag name, a vague error will be thrown at runtime saying something like "Element does not support attachShadowRoot".
These changes add a new diagnostic to the compiler that validates the component selector and gives a better error message during compilation.
PR Close#42245
afd68e5674