feat(schematics): introduce unitTestRunner option to support Vitest

[Tommy228]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/spectator/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

[ ] Bugfix
[X] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

In the schematics, only Jasmine and Jest are currently supported. If no specific option is provided then Jasmine is used by default. If a --jest flag is provided then the schematics will use Jest imports.

Issue Number: N/A

What is the new behavior?

I propose introducing a new unitTestRunner flag and deprecating the --jest flag. I believe this approach is better than adding a new --vitest flag, as it avoids redundancy and streamlines the test runner choice. Please let me know if that sounds good to you.

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

I have tested this locally with Verdaccio.

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