[feature] implement squash function by selecting commits to squash

[FederAndInk]

Issue for https://github.com/Murmele/Gittyup/discussions/434

It would be awesome to have a simple squash functionality by selecting multiple commits. You would select (with shift+click) the commits you want to squash, then squash them.

[Vlod-github]

squash is the basic functionality of working with commits