feat(developer): use json schema to verify source file

As we do not appear to have access to a good DTD validation library in
Node.js, we will instead convert the DTD to a JSON schema file and use
that for validation.

I have not verified every aspect of the JSON schema at this point, but
will incrementally do so as I implement further aspects of the compiler.

This updates all the testing to use the schema. As the schema validation
is slow, I have increased the 'normal' time for tests.