[Blazor] CSS isolation follow-ups

[javiercn]

Tracking issue for follow-ups for CSS isolation.

• Unit tests for concatenate CSS files

• Unit tests for compute css scopes.

• Unit tests for ApplyCssScopes

• Unit tests for discover default scoped CSS items

• Unit tests for ResolveAllScopedCssAssets

• Additional integration tests

  • Disable bundling
  • Disable default scoped css items
  • Ability to plug-in custom scoped CSS item definitions
  • Ability to override scope identifiers
  • Ability to run custom task after bundling.

• Visual studio integration with up to date check

• Packaging validation constraints

• Generated/Sythetic static web assets support

• Support url(relativeToContentRoot) in scoped CSS #24583