GDScript editor helpers

Added:
- Clone Down: Duplicates the current line. (Cmd/Ctrl+B)
- Move Line Up/Down: Move current line up or down, currently it doesn't have a shortcut...
- Indent Left/Right: Indent/Unindent Line (Cmd/Ctrl+Comma/Period)
- Toggle Comment: Comment a Line or Selected lines (Cmd/Ctrl+Slash)
- Some functions like: insert_at() and set_line()

( My first time doing this :P )