(internal, seed): heavy rewrite of seed

[dsinghvi]

This PR pushes a large refactor to seed. In particular, the following things have changed:

• Instead of --workspace we now specify --generator <generator>. So to run tests on the python-sdk you can do yarn seed test --generator python-sdk.
• Instead of a gigantic file with one function, we know have a TestRunner and ScriptRunner. A TestRunner runs the generator and calls into the script runner which runs the scripts. This abstraction paves the path for being able to run seed tests without having to run docker.
• We have 3 commands: seed test to run snapshot tests, seed run to run seed against arbitrary definitions, and seed write-inputs to rewrite inputs like IR / generator Config.
• Parallelization: if you run seed against a single definition, previously that would go generator by generator. Now, it does all of them at once and respects the parallelization flag.