tugger-wix: Add option to add start menu shortcut.

[andrewleech]

This MR adds an msi option to create/install a start menu shortcut for the installed application.

So can be used like:

def make_msi(exe):
    msi = exe.to_wix_msi_builder(
        "wsl_usb_gui",
        "WSL USB",
        VARS.get("version"),
        "Andrew Leech"
    )
    msi.add_to_start_menu = True

Note: My work on the set of PR's I've recently openned here are the first time I've ever used rust... this this PR the biggest. I'm sure some of my variable / mut handling is wrong or at least non-ideal, so certainly open to suggestion on how to clean this up a bit!