feat(@angular/build): add isolate option to unit-test builder

This commit adds an `isolate` option to the `@angular/build:unit-test` builder for the Vitest runner. By default, tests run in a non-isolated environment to match the behavior of Karma/Jasmine. This new option allows developers to easily opt-in to Vitest native isolation (running tests in separate threads or processes) without requiring a custom Vitest configuration file. The option is not supported by the Karma runner and will result in an error if used there.