Comment that anonymous labels internally start with '!'

`startsIdentifier` should not accept this character so
anonymous labels won't conflict with nonymous ones.

src/asm/lexer.c

@@ -1307,6 +1307,7 @@ static uint32_t readGfxConstant(void)
 
 static bool startsIdentifier(int c)
 {
+	// Anonymous labels internally start with '!'
 	return (c <= 'Z' && c >= 'A') || (c <= 'z' && c >= 'a') || c == '.' || c == '_';
 }