feat!: change padding to uniform plus topbar

[AndersDJohnson]

This is a breaking change (so should result in a major version bump to 3.0) for the padding prop to only support single value (.5rem not 2.25em .5em .5em .5em) - it applies uniformly across left, right, and bottom - and on top it is added to the padding needed to account for the topbar height.

Also switching the units from em to rem since otherwise it might not line up since a different font may be applied to the topbar vs. the content (#21).

With this, you can set padding="0" (which I want to do in order to provide custom content with a different background color to the edges) - and not have the content hide underneath the topbar.

Before:

After: