Add Old Turkic layout

Hello,

I have been working on an Old Turkic keyboard layout for a couple of years. After continuous use, I think it's ready to be included in xkb.

I've been using it as a normal layout, not as a variant, but after reading the contribution guide, I made some modifications to it, to make it more appropriate. I've placed it under Turkish for the moment.

I'd appreciate any reviews, probably there are areas to improve in any case.

Thanks!