Added .asize and .isize pseudo variables

These pseudo variables will return the size of the accumulator/index
in bits.

For the 65816 instruction set .ASIZE/.ISIZE will return either 8 or 16,
depending on the current size of the operand in immediate addressing
mode.

For all other CPU instruction sets, .ASIZE/.ISIZE will always return 8.

For example:

	; Reverse Subtract with Accumulator
	; A = memory - A
	.macro rsb param
		.if .asize = 8
			eor	#$ff
		.else
			eor	#$ffff
		.endif
		sec
		adc	param
	.endmacro

src/ca65/scanner.c

@@ -139,6 +139,7 @@ struct DotKeyword {
     { ".ALIGN",         TOK_ALIGN               },
     { ".AND",           TOK_BOOLAND             },
     { ".ASCIIZ",        TOK_ASCIIZ              },
+    { ".ASIZE",         TOK_ASIZE               },
     { ".ASSERT",        TOK_ASSERT              },
     { ".AUTOIMPORT",    TOK_AUTOIMPORT          },
     { ".BANK",          TOK_BANK                },
@@ -224,6 +225,7 @@ struct DotKeyword {
     { ".INCBIN",        TOK_INCBIN              },
     { ".INCLUDE",       TOK_INCLUDE             },
     { ".INTERRUPTOR",   TOK_INTERRUPTOR         },
+    { ".ISIZE",         TOK_ISIZE               },
     { ".ISMNEM",        TOK_ISMNEMONIC          },
     { ".ISMNEMONIC",    TOK_ISMNEMONIC          },
     { ".LEFT",          TOK_LEFT                },