Implement -Wnumeric-string[=0|1|2] (#935)

Fixes #934

test/asm/warn-numeric-string.asm

@@ -0,0 +1,25 @@
+charmap "<NULL>", $00
+
+
+SECTION "ROM", ROM0
+
+MACRO try
+	OPT \1
+	; no warning
+	db "A" * 2
+	db ("<NULL>")
+	; warn at level 1
+	dl ("AB<NULL>CD")
+	dl "<NULL" + ">NULL>"
+	; warn at level 2
+	dl (STRCAT("A", "B"))
+	dl "A<NULL>Z" + 1
+ENDM
+
+	try Wno-numeric-string
+	try Wnumeric-string
+	try Wnumeric-string=0
+	try Wnumeric-string=1
+	try Wnumeric-string=2
+	try Werror=numeric-string=1
+	try Werror=numeric-string=2