Windows IEx enhancements

These commits mainly allow for users to invoke IEx in shells other than `werl.exe` if they so choose.  This is done by providing a new `iexw.bat` which starts a `werl.exe` shell, and having `iex.bat` stay in whatever shell it was called in.

There's also some miscellaneous tweaks:

* Use the `setlocal` directive so that variables used in `elixir.bat` don't linger in the user's environment
* Use the `start` command for calls to `werl.exe` so that the command prompt window doesn't have to stick around
* Hide the "dumb terminal" warning if on Windows (since the concept of TTY doesn't exist in Windows, and since we aren't "falling back" to it)