[numex] Concatenating a string of numeric expressions with no intervening tokens like Seventeen Eighty or Ten Oh Four

src/string_utils.h

@@ -84,6 +84,8 @@ bool utf8_is_punctuation(int cat);
 bool utf8_is_symbol(int cat);
 bool utf8_is_separator(int cat);
 
+bool string_is_ignorable(char *str, size_t len);
+
 bool string_contains_hyphen(char *str);
 bool string_contains_hyphen_len(char *str, size_t len);