caps!: implement explicit width extension

[rockorager]

Implement explicit width hint extension, developed by kitty. When
both explicit width and mode 2027 are available, we default to explicit
width. Custom event loop authors will need to update their loops to add
support for this by setting the new capability value.

For simplicity, we don't actually add a flag in the parser for checking
between a cursor position and an F3 key. Instead, we send the cursor
home, then do an explicit width command, then check the cursor
position. If the cursor has moved - meaning the extension is supported -
we will see an F3 key with the shift modifier. The response will be
something like \x1b[1;2R which we parse as a shift+F3. But in the
loop, we check the flag if we have sent queries and handle this specific
event differently.

Reference: kovidgoyal/kitty#8226