[flutter_migrate] Resolve conflicts command

[GaryQian]

Previously mid-way through review: https://github.com/flutter/flutter/pull/102790

Resolve conflicts wizard. Finds and displays each staged merge conflict and presents options to the developer to accept old, accept new, or skip.

Between each file, the wizard asks developers to commit the changes or not.

This tool exists to help rapidly process routine conflicts that may be generated by the migrate tool. Typical usage is to usually accept the new changes unless something was explicitly manually set. Complex conflicts can be skipped and left to be merged manually.

[GaryQian]

@christopherfujino You previously reviewed the old version of this PR. The suggestions you made there are carried forwards into this one.

[zanderso]

Added @stuartmorgan since he's been reviewing other migrate PRs, and might have the most up-to-date context.

[GaryQian]

@stuartmorgan Addressed comments and ready for another look! Thanks.

[GaryQian]

@christopherfujino Ready for another look!

[reidbaker]

Let me know if you still want this to land. If not we can take it over.

[stuartmorgan-g]

@reidbaker Did you want to take this PR over, or should we close it for now and revisit later?

[reidbaker]

I think close and revisit

[stuartmorgan-g]

Closing; filed https://github.com/flutter/flutter/issues/122305 with a breadcrumb back to this PR for when flutter_migrate is revisited.