This commit upgrades C++/WinRT to 2.0.200316.3 and fixes a couple of the
transitive dependency issues we had in the process.
Because the latest version has better dependency resolution, we're able
to properly depend on Microsoft.UI.Xaml and the Toolkit in TerminalApp
and TerminalAppLib so we no longer need to manually include .dll and
.pri files.
Because of nebulous _other_ changes in dependency resolution,
WindowsTerminalUniversal isn't picking up transitive .winmd dependencies
from TerminalApp, and needs to include them as ProjectReferences
directly. This was already happening transitively, so now it's explicit.
I've also taken the time to upgrade GSL to v2.1.0, the last release
before they removed span::at and blew up our world.
