fix(scanner): address -Wswitch-outside-range warnings when char is unsigned

[jamessan]

C doesn't define the sign of the char type, so some architectures
(x86) use signed char where as others use unsigned char.

On architectures where unsigned char is used or if compiled with
"-funsigned-char", we see the below warnings:

src/scanner.c: In function ‘scn_dir_tag_pfx’:
src/scanner.c:470:13: warning: case label value is less than minimum value for type [-Wswitch-outside-range]
  470 |             case SCN_FAIL:
      |             ^~~~
src/scanner.c: In function ‘scn_tag’:
src/scanner.c:512:17: warning: case label value is less than minimum value for type [-Wswitch-outside-range]
  512 |                 case SCN_FAIL:
      |                 ^~~~
src/scanner.c:526:17: warning: case label value is less than minimum value for type [-Wswitch-outside-range]
  526 |                 case SCN_FAIL:
      |                 ^~~~

Define a new ScanStatus enum to use instead of independent #defines,
and update all the functions which previously returned char to instead
return ScanStatus.