Add a way to tag columns and cells together

[tabatkins]

In w3cping/privacy-threat-model#32, @jyasskin adds some attributes to table headers and table cells to explicitly tie them together, plus some JS to verify the relationship holds, so that any mistakes in placing a cell are caught immediately (rather than accidentally putting a mark in the wrong column).

The precise mechanism they're using is too specialized to generalize, but the capability is doable - tag ths with a name (col=foo?) and then tag tds with the same; if those attributes are present in a table, verify that all tds are tagged and that they're in the appropriate column.