Use `withr::with_seed()` in `random_id()`

[seb09]

Addition or change of position of a gt should not change the state of the random number generator in the global environment. Fixes #1915

Summary

Thank you for contributing to gt! To make this process easier for everyone, please explain the context and purpose of your contribution. Also, list the changes made to the existing code or documentation.

Related GitHub Issues and PRs

• Ref: #1915

Checklist

• [x] I understand and agree to the Code of Conduct.
• [x] I have listed any major changes in the NEWS.
• [x] I have added testthat unit tests to tests/testthat for any new functionality.

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