bug#55722: Missing number superscripts in the `C-x 8 ^' map

[Eli Zaretskii]

> From: Tor Kringeland <tor.kringeland@ntnu.no>
> Date: Mon, 30 May 2022 09:19:37 +0000
> 
> `C-x 8 ^ 1'--`C-x 8 ^ 3' are supported, but not the other seven numbers;
> this should be extended.  Maybe superscripts of + and - could also be
> included --- they are included in some other input methods in Emacs
> (e.g., `compose').
> 
> Also, it would be nice to have the same characters for subscripts (with
> `C-x 8 _').

I added those, but note that the results might look somewhat ugly if
your default font doesn't support the U+20XX codepoints, and you need
a superscript that mixes both Latin digits and the newer 20XX ones.