enhancement: add option for using Powershell as shell

In neovim/neovim#36881, documentation was added to show how to use powershell.exe (or pwsh.exe) as the shell. I have ported the VimScript to Lua:

if vim.fn.has("win32") == 1 then
  vim.o.shelltemp = false
  local shellcmdflag = "-NoLogo -NoProfile -ExecutionPolicy RemoteSigned -Command "
  shellcmdflag = shellcmdflag .. "[Console]::InputEncoding=[Console]::OutputEncoding=[System.Text.UTF8Encoding]::new();"
  shellcmdflag = shellcmdflag .. "$PSDefaultParameterValues['Out-File:Encoding']='utf8';"
  vim.o.shellpipe = "> %s 2>&1"
  vim.o.shellquote = ""
  vim.o.shellxquote = ""

  if vim.fn.executable("pwsh.exe") == 1 then
    vim.o.shell = "pwsh.exe"
    shellcmdflag = shellcmdflag .. "$PSStyle.OutputRendering = 'PlainText';"
    vim.env.__SuppressAnsiEscapeSequences = "1"
  else
    vim.o.shell = "powershell.exe"
  end
  vim.o.shellcmdflag = shellcmdflag
end

The issue names no file or test; start by locating where kickstart.nvim configures the shell and compare that entry point with the supplied Lua snippet. The change is done when users have an option to use powershell.exe or pwsh.exe with the described settings, and the configuration remains valid on other platforms.