[WIP] HTMLBars attributes syntax

Still a WIP, but brings in a few tests and a harness for upcoming complexity.

TODO:

* [ ] Sanity check
* [ ] Port `bind-attr`, maybe general attribute bindings over to use descriptors
* [ ] Is the name "descriptor" dumb?
* [ ] Add tests asserting boolean attrs list

And this lays out a plan for some other stuff:

* [ ] Update HTMLBars to use `classList`, use it here
* [ ] Make `concat` happen in the `lazy` descriptor, since `class` and some others may not want it anyway