Abstract key parsing

[noelwelsh]

We don't currently provide any code to parse key events. This forces users to write their own code to to e.g. interpret up and down arrow keys correctly. This is tedious and error prone. Like most things involving the terminal, key presses are a bit of a mess. We'll probably iterate several times on this before we a solution that works in most cases (and we will need non-blocking read operations).

Here are some resources:

• https://github.com/Textualize/textual/blob/6bc95017a58a8461c0b48bef3fae2dc308e2fe64/src/textual/_ansi_sequences.py lists the sequences that Textual parses
• https://sw.kovidgoyal.net/kitty/keyboard-protocol/ describes an extension that fixes many of the problems with the baseline terminal key handling
• https://vt100.net/emu/dec_ansi_parser describes a parsing algorithm, from the terminal (not application) side.