Fix build with -std=c11

[wezm]

When building with gcc or clang passing -std=c11 scanner.c fails to build (helix builds grammars with this argument):

gcc:

src/scanner.c: In function ‘match_escape’:
src/scanner.c:70:30: warning: implicit declaration of function ‘isascii’ [-Wimplicit-function-declaration]
   70 |                         if (!isascii(lexer->lookahead) ||
      |                              ^~~~~~~

clang:

src/scanner.c:70:9: error: call to undeclared function 'isascii'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
                        if (!isascii(lexer->lookahead) ||
                             ^
src/scanner.c:81:9: error: call to undeclared function 'isascii'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
                        if (!isascii(lexer->lookahead) ||
                             ^
src/scanner.c:92:9: error: call to undeclared function 'isascii'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
                        if (!isascii(lexer->lookahead) ||
                             ^
src/scanner.c:126:9: error: call to undeclared function 'isascii'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
                        if (!isascii(lexer->lookahead) ||
                             ^
4 errors generated.

It seems that isascii is deprecated and is not visible by default with -std=c11. This PR attempts to address that although I'm not 100% sure it's the right fix. A better option might be to define isascii in this project if it's not already defined. Open to suggestions

[fweimer-rh]

I submitted a different fix as helix-editor/helix#8953.

[gdamore]

This is not appropriate because _XOPEN_SOURCE is only for POSIX systems, and this may be used e.g. in Windows.

A simple solution could be to add a local definition:

#ifndef isascii
#define isascii(c) (((c) & 0x80) == 0)
#endif

Put that below the header includes.

[gdamore]

Note that there are no cases for Unicode where the above definition of isascii is incorrect. There are very few other encodings that don't meet this, and we don't support any of them. (E.g. EBCDIC or SHIFT-JIS.) All the Latin-X versions for example meet this criteria.

[gdamore]

I'm closing this in favor of #22 which fixes the same bug.

[wezm]

Cool, happy to have a fix either way.