Add Slider package

[lucat1]

This PR adds the slider to this collection of Material Design components. 🚀

The Slider has the following functionalities:

• Can be dragged anywhere

• On focusing, it can move by using the arrows on the keyboard

TODO: 📝

• [x] Fix thumb from going outside from the slider

• [x] Add more keyboard events

• [x] Fix some movement issues

[lucat1]

Fixed merge conflicts. Moving onto these: 📝

• [x] Add discrete indicator

• [x] Test with values higher than 100

• [x] Fix mouse focus/blur outside of the slider range

[lucat1]

LGTM 👌