Configure TypeScript for Vitest testing globals

[Klerith]

Updated TypeScript configuration to use Vitest globals for testing, allowing usage of testing functions without imports.

PR Checklist

Please check if your PR fulfills the following requirements:

• [ x] The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ x] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [x ] Documentation content changes
• [ ] angular.dev application / infrastructure changes
• [ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Users reading the migration documentation from Karma Jasmine to Vitest will have the global definitions instead of importing them manually.

Does this PR introduce a breaking change?

• [ ] Yes
• [ x] No

Other information

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[SkyZeroZx]

Hi, Could you please sign the CLA (using the email associated with your GitHub account) and amend your commit to follow the commit guidelines

It should look something like this:

docs: Configure TypeScript for Vitest testing globals