add bash-like autocomplete mode

[rpaciorek]

• Add autocomplete with list of suggestions mode (default Ctrl+Tab):
  • If the complement is unambiguous, then a single keystroke causes the complement.
  • If the complement is ambiguous, then a single keystroke does nothing; the next keystroke displays a list of suggestions.
• Use is_action_pressed with exact_match=true for all autocomplete actions
  • This allow the user to remap this mode to Tab and cycle mode to Ctrl+Tab
• Add print_in_columns function for printing list of suggestions
  • It print single line, but due to RichTextLabel this ensures the correct column arrangement even when changing the window size.
• Add _add_input_action in plugin.gd to remove code duplication.

[limbonaut]

Sorry for untimely response. This adds something we already do (but differently), so I don't think I can accept it. Alternative implementations like this would make it harder to maintain. Could use some kind of plugin system for such.