autosave file

When running a regular python file, the file will be saved before the interpreter is started (there is an option in thonny to do this automatically or pop up a warning window).

When uploading to micro:bit, the script is not auto-saved, so it is very easy to upload old versions of the script. Would be nice to not have to spam ctrl-S all the time while editing or getting very confused by things not working as they should on the micro:bit!