Adds a collection of `.pyi` files that describe the expected type
signatures of the publicly available `humanize` functions.

A `MANIFEST.in` file has been added to include the new `.pyi` files and
the `py.typed` file to indicate to mypy that this package supports
typing.

mypy has been added to the pre-commit configuration to check the stubs.

Some of the signatures are slightly optimistic, since depending on error
conditions, different types are returned.

It is better to include typing hints directly in the functions that
they're describing; which allows mypy to actually check the functions,
both in their execution, and that they are actually returning the
appropriate values.

The major change here is that previously a number of the functions, if
they encountered a conversion error, would return the value unchanged.
However, that would require just declaring the return type as Any, which
although correct, isn't particularly useful.

So instead, any failed, unconvertable, inconsistent values will just
have `str` called on them, and the output will returned. The function
will always return a string, although sometimes it won't be in the
format you expect.

In addition, some internal variables inside the function have been
rearranged and moved around, to avoid changing the type of an already
defined variable.

Originally only included this file to include the `.pyi` files in the
package, but it turns out it was redundant, and we don't have `.pyi`
files anymore.

Instead of using `typing.Literal`, which was only added in Python 3.7,
make the type annotations strings, preventing their execution in the
interpreter, and use `typing_extensions` to provide Literal.

In order to make the typing accurate, the failure case was changed in
multiple functions from "return the object unchanged", to "return the
string representation of the object".

As such, a number of tests all needed to be updated.