Validate incoming jsonschemas from Vega-lite within tests

[mattijn]

As was pointed out here #2771 (comment) it is recommended to validate the incoming static jsonschemas:

The schema is included within Altair so it make sense to validate the vega-lite schema during development. Then we can catch these issues before it is shipped. Maybe we can set up a test for this within Github Action.

Yes! This is the thing I usually recommend -- if you have a static schema which you think is supposed to be valid, validate it in CI, then you'll know if something's wrong, but otherwise the library proceeds as normal.

check-jsonschema could be used here, as is described in this comment: #2771 (comment)

[binste]

check-jsonschema 0.23.0 is out now, see python-jsonschema/check-jsonschema#231 (comment). it has support for selectively disabling format checks which is what we need here. I'll work on this soon, feel free to assign to me if it's helpful.