Terminal: ConPTY Passthrough mode

I've discussed this a couple times on different threads before, but I think I never made a real issue for it.

The idea of conpty passthrough mode is that a commandline client application that _knows_ it's only going to use VT sequences (and __not__ the API) to modify the console could set a special mode, ENABLE_PASSTHROUGH_MODE. If conpty is active, the console would then _stop_ rendering itself over conpty, and anything that was written to the console would go straight to the terminal.

• This would work especially well for something like wsl, where it's only ever going to be talking VT.
• It would _not_ work for cmd.exe, because of cmd.exe's heavy reliance on the API.
• Windows Powershell again needs the API pretty heavily, but maybe Powershell Core, which is cross-platform, could make use of it.

This has some rough edges that need to be sorted out.

• [ ] What happens when the app exits, and passthrough mode is turned off? The terminal and conpty's buffers would not be equivalent anymore!
  
  
  
  • I've been toying with the idea of having conpty both pass the sequences through, and also process them itself, so that the terminal and conpty stay in sync.
  
  
• [ ] What happens when someone tries to call Console API's in passthrough mode?
  
  
  
  • I'm thinking we just cause them to fail. All save for Get/SetConsoleMode and reading input / writing output.
  
  
  • We'll also probably need to be able to read resize events.
  
  

This needs a real spec written, but it probably needs prototyping done before that.