Unicode 13 sextant coverage

[dankamongmen]

I notice that Hack supports the quadrant box-drawing characters from Unicode 3.2. Unicode 13 introduced a set of sextant characters in its Symbols for Legacy Computing block. It would be great if these were also present in Hack.

I'm happy to take a go at adding them myself, as they're simple as can be. I wanted to go ahead and file and issue before doing so, in case someone else is already on top of this, or there's some reason not to add them.