Allow passing --changed cli option to vitest

### Command

test

### Description

The `unit-test` builder does not provide a way to use the `--changed` vitest option: https://vitest.dev/guide/cli.html#changed. This will analyze the import graph and only run tests affected by a change, which is useful for running a subset of the test suite for pull requests. This option is not available through the vitest configuration file.

### Describe the solution you'd like

Add a `changed: ` option to the `unit-test` builder options which passes `--changed ` to the vitest runner.

### Describe alternatives you've considered

I don't think there are any good workarounds. Using `@analogjs/vitest-angular` exposes vitest directly, but requires more setup and is not an "official" package.

Beginne bei den Optionen des Unit-Test-Builders und verfolge, wie sie an den Vitest-Runner weitergegeben werden. Überprüfe, dass eine geänderte String-Option --changed weiterleitet und dass die relevanten Builder-Tests den daraus resultierenden Befehl abdecken.

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