Integration of tex-mode into texinfo-mode

[Christopher Dimech]

Dear Compeers,

Recently texinfo was updated to allow html output using mathjax.  The problem 
with
texinfo-mode highlighting is that tex commands can be introduced in a texinfo 
file
using @math, @displaymath, and @tex.  Yet  texinfo-mode does not highlight tex 
commands
as that provided by tex-mode.

The next iteration of texinfo-mode should also incorporate tex-mode for tex 
command
regions.  A very helpful feature of tex-mode in the different face given to 
superscripts
and subscripts.

Regards
Christopher