Improve dotnetup shell profile UX and transparency

## Summary

Follow-up from the PR review on #53447.

The current shell-profile flow works, but it still hides too much of what dotnetup is about to edit. We should make the experience more explicit and give users more confidence about what will happen before their shell profile is modified.

## Proposed work

- show the exact shell/profile file(s) dotnetup plans to modify before writing them
- consider adding a confirmation prompt/consent step when profile files will be edited
- evaluate whether we should support updating more than one detected/supported shell profile instead of only the current shell
- revisit whether sourcing a managed file is preferable to eval / Invoke-Expression in the long term
- consider whether the profile update flow should use a safer temp-file/replace pattern when modifying existing files
- Update "Shell profile is already configured." message to include path to profile file.

## Related

- #53447
- #53838