Convert messages to f-strings and overides to list + other minor edits

[bergamote]

Hi, your script is very useful! Reading through it I thought that some parts could be made a bit more readable so here's what I did:

• Added a header line variable h_line = 80 * "#" since it was used often
• Converted the printed messages to triple quoted f-strings, since the code has the same 80 char limit as the terminal it makes them easier to read and to edit in the future
• Converted blendfile_override_setting from a string to a list that is then joined before being written to file, again for readability but also because it's recommended in PEP 8
• Added a standard cancel key "C", which is then changed to "BREAK" for Windows
• Removed pathlib import Path since we already import os we can use os.path.exists()
• Changed some simple expressions and fixed a few typos (very little)

I paid attention to not change the script's output or touch any of the functionalities, so apart from a couple of messages where I had to move a line break by a word or two, everything should be exactly the same from the user point of view.