Add CompatGroup macro

[wbamberg]

This PR adds a new macro CompatGroup. This macro takes as an argument the name of a CSS module (e.g. "CSS Animations"). It uses https://github.com/mdn/data/blob/master/css/properties.json to fetch the properties in that group, and renders a table containing JSON data for those properties. This enables us to embed a single table in pages like https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Grid_Layout that gives an overview of browser support for that area.

To do this is also splits the existing CompatBeta macro. At the moment this macro takes a path through the compat data object as an argument, resolves that into an array of JSON compat objects, then renders those objects.

To enable CompatGroup to share the rendering code (which is most of the work) this PR also splits out the rendering work from CompatBeta into a new macro CompatRender.

So now we have:

1. CompatRender: given an array of JSON objects, one for each table row, render a table
2. CompatBeta: given a path through the JSON data, get the JSON objects, flattening them if needed, then call CompatRender
3. CompatGroup: given the name of a CSS module, get the JSON objects for the properties it contains, then call CompatRender

The macro is called CompatGroup even though it's much more specific than this, currently only dealing with CSS modules. Maybe in future it could do a similar trick with JS and Web APIs, but we'd need solid metadata first.

I've done some manual testing to try to confirm that we haven't regressed the existing Compat macro with this change.

[wbamberg]

I've added some basic tests for this macro. Would love feedback on whether this is a good way to structure tests and whether there are any conditions I've missed that I should have got.

We should probably also have tests for CompatRender as well, I guess.

[a2sheppy]

Would be lovely to see progress on this but am not sure if or how I can be of any help; I'm eager to see this eventually expand to API documentation.

[wbamberg]

It doesn't seem likely this will get merged.

[Elchi3]

For the record: A more recent idea to maybe achieve this was notion to add keywords to things in BCD (or to stumptown). See this great comment by Will: mdn/browser-compat-data#4051 (comment)