Rule 000-US-spellcheck uses en_US dictionary that has both US and UK spelling

Right now, we have the US spelling dictionary that contains the UK spelling variants too.

See [example](https://github.com/canonical/documentation-style-guide/blob/main/styles/config/dictionaries/en_US.dic#L51838).

So the option in the [rule itself](https://github.com/canonical/documentation-style-guide/blob/main/styles/Canonical/000-US-spellcheck.yml#L12) to enable the UK spelling dictionary doesn't make sense.
Let's replace the single dictionary with two (US and UK), so everyone can use whatever dictionary they prefer.