Update Autoconf lexer grammar:
- Add m4sh AS_macros.
- Update definition of A{C,H,M,S}_* generic macros.
- Add definition of m4_* macros.
- Add definition of a{c,h,m,s}_* generic variables.
- Update definition of _AC_* internal macros.
Add definition for ("_")?"A"["C","H","M","N","U","X"]"_" macros and variable types.
Add definition for ("_")?"A"[${A}]"_" macros and variable types where ${A} is any character not explicitly defined in previous rules.
Update Autoconf lexer grammar.