Add core functionality to default intern export

[jason0x43]

Core functionality, such as the built in test interfaces and assertions, should be available to import through the base Intern export.

import { describe, it } from 'intern';
describe('something', () => { ... });

At least the test interfaces (object, bdd, tdd), assert, and expect should be exported. The implementer should also verify that the necessary types are exported so that the package is usable when installed.

See #1041 for discussion

[jason0x43]

Another reason for this -- when directly importing interfaces, you'll end up building parts of intern into tests. That can lead to subtle issues.