* set identifying environment variable for new connections
Set a new 'WT_SESSION' environment variable when creating new terminal
connections to allow shells to detect a unique Windows Terminal session.
The value of the variable is a stringified GUID as returned by
CoCreateGuid.
How verified:
- "razzle" & vs debug build
- runut
- manual inspection
* * use winrt::guid type for connection guid
* use Utils::GuidToString for guid stringification
* expose guid parameter in ITerminalConnection idl
* - poke guid through ITerminalConnection
- misc. review fixes
- throw if CreateConPty fails in ConhostConnection::Start
- apply [[nodiscard]] and noexcept in various places
* - simplify environment variable extraction in UpdateEnvironmentMapW
* - use Utils::CreateGuid instead of CoCreateGuid in ConHostConnection()
