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/scanner.c

@@ -260,10 +260,12 @@ struct DotKeyword {
     { ".PARAMCOUNT",    TOK_PARAMCOUNT          },
     { ".PC02",          TOK_PC02                },
     { ".PDTV",          TOK_PDTV                },
+    { ".POPCHARMAP",    TOK_POPCHARMAP          },
     { ".POPCPU",        TOK_POPCPU              },
     { ".POPSEG",        TOK_POPSEG              },
     { ".PROC",          TOK_PROC                },
     { ".PSC02",         TOK_PSC02               },
+    { ".PUSHCHARMAP",   TOK_PUSHCHARMAP         },
     { ".PUSHCPU",       TOK_PUSHCPU             },
     { ".PUSHSEG",       TOK_PUSHSEG             },
     { ".REF",           TOK_REFERENCED          },