Fix incorrect lexing of "$ff00+c" (#882)

Fixes #881 by moving the task from the lexer to the parser.
This both alleviates the need for backtracking in the lexer,
removing what is (was) arguably a hack, and causes tokenization
boundaries to be properly respected, fixing the issue mentioned above.

Co-authored-by: Rangi <remy.oukaour+rangi42@gmail.com>

test/asm/ff00+c-bad.asm

@@ -0,0 +1,9 @@
+
+SECTION "ff00+c or not to ff00+c", ROMX
+
+	ld a, [$ff00 + c]
+	ld [65280 + c], a
+
+	; Not ok
+	ld a, [$ff01 + c]
+	ld [xyz + c], a