Add a github commit test to check header files for missing extern "C"

[peterharperuk]

I suggest something simple to be going on with...

grep -r -L '^extern "C"' . --include '*.h' | grep '/include/pico/'

Find header files in "/include/pico/" that don't contain extern "C". This will catch the worst cases?
Just throw an error if the list of offending headers changes?

[peterharperuk]

@lurch You know how to do this - right?

[lurch]

@will-v-pi has more familiarity with GitHub CI than I do 🙂 But I can certainly take a look at this next week.