Add uv build option for template creation

[Dhananjhay]

Proposed changes

This PR adds support for the uv package manager as an option when creating a Snakebids app using the snakebids create command.

Key changes include:

• Conditional logic in the Jinja template files to configure the pyproject.toml and install instructions for uv
• Logic alignment with uv_build for backend config (i.e., uses uv_build as the build backend for uv)
• Updated CLI validation logic to recognize uv as a valid option

This resolves support request/feature expansion for modern Python packaging tools and enables compatibility with uv.

Checklist

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• Changes have been tested to ensure that fix is effective or that a feature works.
• Changes pass the unit tests
• I have included necessary documentation or comments (as necessary)
• Any dependent changes have been merged and published

Notes

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[Dhananjhay]

@pvandyken I ran some local tests and found out some bugs which I just patched in the latest commit. Once you re-review this PR again, I can merge it in the main. Thanks!