Align calcite-panel to bottom of the screen

[BenPitcairn]

The `action-bar` has a slot for actions to be placed at the bottom. I would like to have the panels that are toggled by these actions appear at the bottom of the screen as I feel like that makes intuitive sense for a user. Ideally I would be able to have two panels open on one side (within the same `calcite-shell-panel`), one aligned to the top of the screen and one aligned to the bottom.

[MattDriscoll]

Great suggestion @BenPitcairn . We can do that by adding some new slots for center content at the 'start' (top) and 'end' (bottom).

Created issue here: https://github.com/Esri/calcite-components/issues/5668