Model Discovery

[marvinirwin]

Adds the command lb4 discover command and its documentation.

The command has a schemaDefs option (the path to a file which contains a schema definition, or array of lb3 schema definitions) which overrides everything and simply creates the corresponding typescript file. It's used for the integration test, and could also solve #2224

It's a piece of #2090
Implements #1921

Checklist

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated