Feature request: #elifdef and #elifndef from C23

[peterjc]

The code in https://github.com/ned14/pcpp/blob/master/pcpp/preprocessor.py doesn't yet cover #elifdef and #elifndef which are new in C23.

Reported upstream as caksoylar/keymap-drawer#202

[ned14]

I was one of those who voted that into the C standard at the time! :)

I'll add it to the todo schedule. Thanks for the suggestion.