Mark `unit-test` builder's `runnerConfig` option as experimental

### Command

test

### Description

Considering the way `unit-test` builder works, not all Vitest config options are available.
Some options are explicitly detected and the user is warned:
https://github.com/angular/angular-cli/blob/720603223f2e9d1487ce54dac903fe8954bda3a5/packages/angular/build/src/builders/unit-test/runners/vitest/plugins.ts#L81

But there are also some options or configurations that could cause surprising issues.
For example, as of 21.0.0-rc.3, `isolate: false` can break the tests in browser mode.

### Describe the solution you'd like

It would be nice if the option was marked as experimental or at least in dev preview and/or if it would come with a warning.
It would set the expectations right and leave some time for the Angular team and the community to document or fix the rough edges.

### Describe alternatives you've considered

In the future `@angular/build` could either:
- expose a `defineAngularVitestConfig` function that only allows the subset of Vitest options that are known to be supported by the CLI.
- expose a Vitest plugin and give full control for most advanced configs and use cases