The current handling of preprocessor directives in filetype/c-family.kak
leads to a wall of solid colour for more complicated #if or #define
directives, although #include is already nicely highlighted.
Instead of highlighting an entire directive with the meta face, highlight
just the #define, #if or #elif keyword as meta, treating the rest of the
directive as normal c-family expressions. This significantly improves
the readability of complex macro definitions.
For directives other than #define, #if and #elif, we treat the rest of
the directive as an opaque string in normal face rather than trying to
highlight it, covering cases like #error, #pragma, etc. where the rest
of the line is an error message string or other non-expression content.
This does the right thing for #ifdef and #ifndef too, as we don't highlight
identifiers in c-family text so their arguments should be normal face
anyway.
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