feat: theme & content now a plugin

[Ademsk1]

Fixes #178

Plugin imported via require('@nearform/quantum/plugin'). Tested on a separate repo using yalc and results seemed good, required me to pass in our package to content. Pushing our package into content doesn't seem possible. Here were my findings

UPDATE
Was able to push to the config easily. It seems the config() actually returns the consuming applications object by reference, and so was able to modify it as such.

~~1. Adding the content key into the plugin was unsuccessful for a variety of different paths (I assumed there may be some dynamic imports necessary, tried them all and no luck).
2. Several of the big component libraries using Tailwind (Flowbite, Preline) require you to add in the package as content manually.
3. The Plugin page does not mention content being modifiable anywhere, however presets does. It first seemed like a better avenue to go down, as the docs quote about presets:

"This can be very useful for teams that manage multiple Tailwind projects for the same brand where they want a single source of truth for colors, fonts, and other common customizations."

However, if one scrolls down to to the merging logic section , you can see that the consumers content would overwrite our own. I've raised a discussion about other avenues to resolve this here. ~~

[review-bot-for-github]

Looks good! No comments.

[nstolpe]

This looks good. I found one unimplemented test that could use some logic (or maybe removal?) and a few comments that look like they could be removed.

[nstolpe]

👍