For the first non-nested matching pair of `a` and `b` in `str`, return an
object with those keys:
-**start** the index of the first match of `a`
-**end** the index of the matching `b`
-**pre** the preamble, `a` and `b` not included
-**body** the match, `a` and `b` not included
-**post** the postscript, `a` and `b` not included
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', '}']`.
### const r = balanced.range(a, b, str)
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]`.