feat: optional timeout on pattern mode

[ArijanJ]

I really liked Emacs' avy-goto-char-timer, and this seemed like the best project to add this feature to in the Neovim space.

Basically, it's the regular pattern search, but with an automatic enter press after a certain time of no typing. Here's a little demo:

https://github.com/smoka7/hop.nvim/assets/56356662/85d11065-f8e0-49de-bb33-3aafbfbfb1f9

I had to change the getcharstr call to always return something each cycle instead of wait for a key to be pressed, which meant a few more conditionals, but I think it should be fine.