Support `noEmit` in OpenAPI 3 emitter

[wanlwanl]

Clear and concise description of the problem

Support noEmit in OpenAPI 3 emitter like

typespec/packages/protobuf/src/lib.ts

Line 35 in 9181b54

noEmit: {

FYI. E2E sample needs it.

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[wanlwanl]

Already supported