[App Service] Add a short name alias for the slot parameter

[lostintangent]

Numerous commands within the az appservice web group accept a --slot parameter, and it would be great if it was aliased (perhaps as -s) to enable more efficient usage.

Good examples of where this would help are the az appservice web browse command or the az appservice web deployment slot swap command, both of which may be used frequently, and would benefit from saving four keystrokes for each execution.