feat(@angular/build): support Istanbul coverage in Vitest runner

This change enables code coverage reporting when running tests in non-Chromium browsers (like Firefox or Safari) with the Vitest runner.

The system now automatically detects the best coverage provider based on the configured browsers and installed packages:
- If non-Chromium browsers are configured, it selects 'istanbul'.
- If only Chromium browsers are used, it selects 'istanbul' if it is the only provider package installed.
- Otherwise, it defaults to 'v8'.
It also respects the provider specified in the user's custom Vitest configuration.