Implemented charmap stack

New commands:
 .PUSHCHARMAP: will push the current charmap state into an internal stack
 .POPCHARMAP: will restore the current charmap to the last pushed charmap

Details:
  The push and pop facilities are implemented directly inside the tgttrans.h,
  to facilitate its reuse on the C compiler.

src/ca65/token.h

@@ -231,10 +231,12 @@ typedef enum token_t {
     TOK_PARAMCOUNT,
     TOK_PC02,
     TOK_PDTV,
+    TOK_POPCHARMAP,
     TOK_POPCPU,
     TOK_POPSEG,
     TOK_PROC,
     TOK_PSC02,
+    TOK_PUSHCHARMAP,
     TOK_PUSHCPU,
     TOK_PUSHSEG,
     TOK_REFERENCED,