Modernize VtPipeTerm (#17647)

This shortens VtPipeTerm quite a bit, which used to have various debug
flags and modes. I kept the `--out` flag to redirect the output to a
file, but I removed the `--debug` (pipe the output through WSL and
show escape sequences visually) and `--headless` (hide conpty) flags.

I did this, because VtPipeTerm always used the system ConPTY API
but I needed it to use my local OpenConsole. I also wanted it to
use overlapped IO for testing but found that it was too difficult
to refactor make that work.

I also noticed that the project was the only holdout for
`conpty.h` which had to be kept in sync with `winconpty.h`.

.github/actions/spelling/allow/apis.txt

@@ -174,6 +174,7 @@ PALLOC
 PATINVERT
 PEXPLICIT
 PICKFOLDERS
+PINPUT
 pmr
 ptstr
 QUERYENDSESSION

.github/actions/spelling/expect/expect.txt

@@ -913,6 +913,7 @@ Keymapping
 keyscan
 keystate
 keyups
+Kickstart
 KILLACTIVE
 KILLFOCUS
 kinda