wt.exe should accept env vars on the commandline

[zadjii-msft]

forked from #15496

wt -E foo=bar to manually pass env vars to the NewTerminalArgs. This'll let a caller manually pass env vars on the commandline. Great!

builds off the work in #15082. Basically, each of the -E foo=bar args would get collected by the arg parser, then eventually passed to TerminalSettings::CreateWithNewTerminalArgs. We'd layer them on top of anything from the profile.

open questions:

• syntax?
• What if I want to pass -E foo=%bar%, where bar is something defined in the profile's env block, but not the current one?
  • IMO we can just say no to this. This seems impossible, and also why?