* add `collapseDuplicateDeclarations`
This will allow us to remove duplicate declarations. This occurs when
you are using `@apply` for example.
The reason I implemented it as a separate step, is because this doesn't
only happen for `@apply`, but it also happens if you do something like:
```js
addComponents({ '.btn-blue, .btm-red': { padding: '10px' } })
```
So instead of tracking down every place this is happening, it now
happens at the very end.
* use new plugin in processTailwindFeatures
* add/update tests by removing duplicate declarations
* update changelog
