feat: add support for string mapping types

[bc-m]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• 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
• Other... Please describe:

What is the current behavior?

String mappings like Uppercase<string> or Lowercase<string> are not recognized as string type.

Issue Number: #2606

What is the new behavior?

String mappings are recognized as string type.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Closes #2606

[bc-m]

Checks should succeed if #2512 has been merged. @kamilmysliwiec Feel free to ping me if I can rebase this.

[kamilmysliwiec]

LGTM