Custom exports for custom aggregation tables about browser-compat-data

[wbamberg]

I think we're definitely going to want some way to group things if we're to be able to build interesting aggregate tables. Also for example think of a table showing compat status for all Grid properties.

But this doesn't seem like compat data, it seems like general metadata, so it might be better to represent it in mdn/data. This data might have other uses than compat tables - for example, a page might want to use it to build a sidebar, or even to build the page's main content.

(For example, a Grid landing page could use mdn/data to get the set of Grid properties, use that to build a sidebar and its content, and also use that data to index into the compat data, to build an aggregate table.)

Does that make sense?