Improve auto-indent

[michielp1807]

Currently the auto-indent just copies the indentation of the current line when pressing enter.

Ideally we would:

• automatically insert an extra tab if a new block is opened (e.g. after a function, if, for, while, etc.)
• automatically dedent after typing end on a new line (unless the user already manually adjusted the indentation, maybe look for the line that opened the block to match the indentation?)
• optionally also make Alt+Up and Alt+Down automatically indent

This would require some basic parsing of lua syntax, see lua manual for syntax definitions