Add first level of ipuz import and export support.

[ianfhunter]

Pretty much modelled off of the existing puz support. Tested locally with some of my own puzzles exported from Exet

Main benefits I see are:

• Cross-compatibility with other apps
• Allow saving of bars, circled cells, etc

Enables future development of:

• prefilled squares
• Diagonal clues (lol)
• unnumbered clues
• Arbitrary numbering
• borderless clues
• arbitrary shapes

and other weird stuff.

For immediate follow-up tasks, I think the following would be useful:

• improved enumeration saving/restoring
• save/restore of shrodinger/alts

[mdirolf]

I've taken a quick look and it looks like a great start - thank you! Once the CI errors are sorted I'll take a deeper look / test locally and then merge.

[mdirolf]

And agreed about supporting alts (and maybe colored grid cells too?) as immediate next steps - would be great to get the full suite of Crosshare features exportable.

[mdirolf]

Thank you!

Will try to get a deploy out ASAP and post here when it's live.