This commit deletes ConhostConnection and replaces it with
ConptyConnection. The ConptyConnection uses CreatePseudoConsole and
depends on winconpty to override the one from kernel32.
* winconpty must be packageable, so I've added GetPackagingOutputs.
* To validate this, I added conpty.dll to the MSIX regression script.
* I moved the code from conpty-universal that deals with environment
strings into the types library.
This puts us in a way better place to implement #2563, as we can now
separately detect a failure to launch a pseudoconsole, a failure to
CreateProcess, and an unexpected termination of the launched process.
Fixes#1131.
357e835f5d