feat(cli): add doom.ps1 for Windows users

c9acdb7 removes doom.cmd because it was broken in most cases. This adds
doom.ps1; an alternative script for Windows+Powershell users, which
properly initializes the state it needs. Naturally, it requires
Powershell 3+ be installed on your systems, but it can be invoked from
either cmd.exe or PowerShell.exe.

This is the first powershell script I've ever written, so I expect edge
cases (for one, shell commands passed to `exit!` will need to be guarded
against the environment).

This also requires emacs.exe be your $PATH, however, unless you set
$EMACS to its path first. E.g.

  $env:EMACS = "C:\Program Files\Emacs\emacs-29.4\bin\emacs.exe"

That said, if you use WSL2, you're still far better off using the bash
script (bin/doom).

Ref: c9acdb72a4

bin/doomscript

@@ -57,6 +57,7 @@ if [ ! -f "$EMACSDIR/early-init.el" ]; then
 fi >&2
 # Some state that Doom's CLI framework needs to know about the terminal. Read
 # the comments at the top of bin/doom for explanations.
+export __DOOMSH="${__DOOMSH:-sh}"
 export __DOOMPID="${__DOOMPID:-$$}"
 export __DOOMSTEP="${__DOOMSTEP:-0}"
 export __DOOMGEOM="${__DOOMGEOM:-$(tput cols 2>/dev/null)x$(tput lines 2>/dev/null)}"