Support short options and handle some edge cases

- Add support for short options (`-a`, `-P 18`, `-6`, etc) using `dargs({ a: true, P: 18, 6: true })`.
Every single-character object key will be considered a short option if it is alphanumeric, or discarded otherwise, e.g. `'-': true` or `'@': 5` will be ignored.

- Trim option names and disallow empty string option names (see https://github.com/sindresorhus/dargs/issues/5).

- Add an option with a `NaN` value to tests.