enh(c) Update keyword list for C11/C18

Pulling this out because we should eventually apply this to our C grammar (removing the C++ keywords and types, etc).

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The full list of keywords for C11/C18 is

                auto                          if                             unsigned
                break                         inline                         void
                case                          int                            volatile
                char                          long                           while
                const                         register                       _Alignas
                continue                      restrict                       _Alignof
                default                       return                         _Atomic
                do                            short                          _Bool
                double                        signed                         _Complex
                else                          sizeof                         _Generic
                enum                          static                         _Imaginary
                extern                        struct                         _Noreturn
                float                         switch                         _Static_assert
                for                           typedef                        _Thread_local
                goto                          union

_Originally posted by @ztane in https://github.com/highlightjs/highlight.js/issues/2973#issuecomment-779284055_

[klmr]

No namespaces, no.

And that begin rule is pure C++, not C.

[klmr]

They’re types, yes, but they’re also keywords (unlike e.g. bool, which in C is an alias for _Bool, but only if the stdbool.h header is included — otherwise it’s just any old identifier). https://en.cppreference.com/w/c/keyword is a good reference.

(Apologies for the lack of activity, I’m still snowed under with two other projects that unfortunately take priority and take up all my nominal “free time”.)