Handle `CSI u` Input (aka fixterms)

[Christian Rocha]

It would be nice for Bubble Tea to support the "CSI u" Encoding Specification as outlined in Paul Evans' seminal work "Fix Keyboard Input on Terminals - Please".

References:

• Wezterm
• Kitty
• Windows Terminal
• iTerm2

Special thanks to @imsnif for the suggestion.

[Wes McNamee]

Yes please!

[Christian Rocha]

Closing this in favor of #869 which is generally compatible with fixterms, and builds on it.