feat: split the folder actions out of the repository extension

Open Terminal Here and Open in Editor were bundled with git emblems, columns and a properties tab, but neither of them has anything to do with git and both work on any folder. They are their own plugin now, on the nemo-repo-core config and launcher layer.

The editor entry used to be shown only when Visual Studio Code was installed, whatever editor_command said; it now checks the configured command. The terminal entry is hidden when no emulator is present instead of appearing and doing nothing.

.gitignore

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