utils/github: add create_bump_pr

[SeekingMeaning]

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This adds an open_pr method that will be useful for brew bump-cask-pr (#7986)

I currently have open_pr in dev-cmd/bump-formula-pr.rb to keep the diff clean — I'll probably move it to utils/github.rb once this is approved

[MikeMcQuaid]

Makes sense to me, thanks!

[SeekingMeaning]

• Inlined determine_remote_and_username
  • Moved this logic back to before git fetch --unshallow (before bump-formula-pr: make --dry-run conflict with --write #8606) so that the formula/cask can be restored if fork creation fails