Allow using custom 'dark' class

[csansoon]

Describe the feature in detail (code, mocks, or screenshots encouraged)

ModeWatcher is designed to automatically add and remove a dark class to the html element, facilitating theme management in conjunction with Tailwind.

However, Tailwind supports custom prefixes for its classes, which also requires using this prefix to the class used to detect dark mode. To address this, introducing a property in ModeWatcher that allows users to specify the class for dark mode detection could resolve the problem. This enhancement would ensure compatibility regardless of the class naming conventions used in Tailwind CSS.

Example

<script>
  import { ModeWatcher } from 'mode-watcher'
</script>

<ModeWatcher customClass='custom-dark' />

What type of pull request would this be?

New Feature

Provide relevant links or additional information.

https://tailwindcss.com/docs/dark-mode#toggling-dark-mode-manually