feat(rules): add trailer-exists rule for commitlint v13

Author: jdbruijn

src/rules.ts

@@ -31,6 +31,7 @@ const rules: Plugin['rules'] = {
   'function-rules/subject-full-stop': functionRule as Rule<unknown>,
   'function-rules/subject-max-length': functionRule as Rule<unknown>,
   'function-rules/subject-min-length': functionRule as Rule<unknown>,
+  'function-rules/trailer-exists': functionRule as Rule<unknown>,
   'function-rules/type-case': functionRule as Rule<unknown>,
   'function-rules/type-empty': functionRule as Rule<unknown>,
   'function-rules/type-enum': functionRule as Rule<unknown>,