symbols/us: add dvorak-altgr-intl layout

This is a relatively common user layout that I have seen many people create for themselves locally on Windows but never see anyone propose officially for Linux/Unix.

Example 1: http://arjenvankol.com/dvorak.php Example 2: http://keyboards.jargon-file.org/#USID Example 3: https://github.com/grilix/dvorak-intl-windows My own attempt: https://github.com/jmcwilliams403/dvorak-altgr-intl-extd

The appeal is obvious: Being able to type monolingually undisturbed while still having access to the international symbols via AltGr.

The approach here to creating a Dvorak analogue of the us-altgr-intl layout is a bit more conservative, changing only the bare minimum to get the dead keys off of the bottom two levels, but a single liberty has been taken to adopt us-altgr-intl's clever swapping of onesuperior and exclamdown, as well as taking it a step further and also swapping dead_hook with questiondown to match.