[VSCode] Support first party user to emit code for different languages

[chunyu3]

TypeSpec extension emit code for first party:

1. provide the default azure emitters for data-plane and manage-plane (should be different with 3rd party) when configure the emitter
2. Be able to use client.tsp or main.tsp as entry point to emit code.
3. Emit different language code

• emit .NET code successfully.(with out .NET SDK repo?)
• IDE should emit Java, Python, Go code successfully
• IDE should emit JS code successfully with different scenarios: (JS RLC, Modular)
• IDE should use autorest emitter to generate Swagger 2.0 code.

4. Preview API documentation should generate swagger 2.0 for Azure scenarios. (autorest emitter @azure-tools/typespec-autorest)

[chunyu3]

Question:

1. How extension identify it is a azure typespec project or not? Extension need this information to provide correct emitter(Add PR Validation #1 task).
2. How extension identify if it is management-plane or data-plane?

[RodgeFu]

the client.tsp as entrypoint issue can be handled by #6339. We may want to do something in vscode extension to unblock the scenario if #6339 won't be fixed in near future.