If there's no match, `undefined` will be returned.
If the `str` contains more `a` than `b` / there are unmatched pairs, the first match that was closed will be used. For example, `{{a}` will match `['{', 'a', '']` and `{a}}` will match `['', 'a', '}']`.
For the first non-nested matching pair of `a` and `b` in `str`, return an
array with indexes: `[ <a index>, <b index> ]`.
If there's no match, `undefined` will be returned.
If the `str` contains more `a` than `b` / there are unmatched pairs, the first match that was closed will be used. For example, `{{a}` will match `[ 1, 3 ]` and `{a}}` will match `[0, 2]`.