Lex $ff00+c without needing large peek lookahead

This also allows arbitrary amounts of whitespace in `$ff00 + c`,
instead of needing to fit in the 42-byte LEXER_BUF_SIZE

test/asm/ff00-plus-c.asm

@@ -0,0 +1,5 @@
+SECTION "test", ROM0[0]
+	ld [ $ff00 + c ], a
+	; 257 spaces exceeds both LEXER_BUF_SIZE (42) and uint8_t limit (255)
+	ld [ $ff00 +                                                                                                                                                                                                                                                                 c ], a
+	ld [ $ff00                                                                                                                                                                                                                                                                 + c ], a

test/asm/ff00-plus-c.out.bin

@@ -0,0 +1 @@
+粐�