Superscript period and superscript slash not defined in the unit registry

It would be really cool if you could support `⋅` and `⸍` for defining units.

## `⋅`

```py
pint.Quantity(0, "ft²·gr⁰⋅³³³³³³/lb")
```

```
UndefinedUnitError: '⋅' is not defined in the unit registry
```

## `⸍`

```py
pint.Quantity(0, "ft²·gr¹⸍³/lb")
```

```
UndefinedUnitError: '⸍' is not defined in the unit registry
```