bug#64212: 29.0.91; \Phi missing in TeX input method

[Eli Zaretskii <eliz@gnu.org>]

> From: Roland Winkler <winkler@gnu.org>
> Date: Wed, 21 Jun 2023 18:24:20 -0500
> 
> 
> The TeX input method is missing \Phi that should give GREEK CAPITAL
> LETTER PHI.  I am surprised I never stumbled upon this previously.  But
> apparently that's what it is (with emacs -Q).

Why do you need that letter in a TeX context?  Greek has its own input
method, and GREEK CAPITAL LETTER PHI is there (type 'F').  AFAIU, the
TeX input method is more for typing scientific text in TeX and its
variants, and there they use a different codepoint for the symbol.

> (There is a special treatment of variants of the greek letter phi in
> latin-ltx.el that goes beyond my knowledge of how input methods are
> implemented.)

If all that's needs to be added to the input method is

   ("\\Phi" ?Φ)

then I can do that.  If something more complex is required, patches
are welcome, as I don't use this input method (nor TeX in general).

I added Stefan who I think knows more about this.