Parsing unit fails with parentheses and exponents; documentation unclear

**Describe the bug**

When trying to parse the unit `lbf/(mile/h)^2` a SyntaxError is thrown.

`SyntaxError: Unexpected "^" in "lbf/(mile/h)^2" at index 12`

This is unexpected since the unit should be valid and math.evaluate() can parse it without issue (see interactive prompt on https://mathjs.org/)

Narrowing down the issue it seems to be related to the `^` after the brackets.

**To Reproduce**

These tests reproduce the issue.

**The original unit**
```js
it('should convert a unit with squared brackets (original)', function () {
const u = Unit.parse('lbf/(mile/h)^2')
assert.notEqual(u, null)
})
```

**Only squared brackets:**

```js
it('should convert a unit with squared brackets (reduced)', function () {
const u = Unit.parse('(h)^2')
assert.notEqual(u, null)
})
```