bug#54027: Wishlist: Support full CSI u specification for terminal input

[Eli Zaretskii]

> From: Alex Hutcheson <alexhutcheson@google.com>
> Date: Sun, 27 Feb 2022 13:21:20 -0500
> Cc: 54027@debbugs.gnu.org
> 
> We basically need to support the cross-product of:
> modifier combinations x ASCII characters

No, AFAIU we only need to support keys+modifiers that are not
otherwise supported already.  E.g., C-a is already supported, so we
don't need to add it, and similarly many other combinations are
already supported.  Or what am I missing?