What to do with "demo"?

Demo is an interesting bit of the schema. Right now it's only referenced from `CustomElement#demos`, but really, anything could have an associated demo, including or especially the package as a whole.

We could add a `demos` field to all declarations, modules and package. We could also add an extensible `metadata` field to just about everything and make `DemoMetadata` well-known object that can appear there.

The issue only points to CustomElement#demos and offers two competing schema directions: demos on more entities or extensible metadata with DemoMetadata. Start by reviewing the existing schema around CustomElement#demos; no files or tests are named, and done requires a decided direction plus a clearly specified schema change.

Written by the indexing model from the issue text.