refactor: rebuild as a git menu plugin on nemo-repo-core

Keeps the git half of the context menu from the single file extension. Open Terminal Here and Open in Editor are not git actions and work on any folder, so they moved to nemo-folder-tools.

The actions are a table rather than three near identical blocks, which is what makes adding git fetch a one line change. Entries now also appear inside a repository rather than only on its root, using the upward walk from nemo-repo-core.

.gitignore

@@ -0,0 +1,2 @@
+__pycache__/
+*.py[cod]