add-models-subpath-in-modular-generation

[qiaozha]

This PR is meant to reflect the suggested changes on adding a models subpath export in the modular generation.
In the previous implementation, we generated src/api/models.ts and export all models to both the api layer index.ts, and the classical client layer.
This PR:

1. moves all the models into src/models/models.ts, and all the operation options into src/models/options.ts.
2. removes the export for models in api layer index.ts
3. changes all the places that refer to models and operation options.

fixes #1913

[qiaozha]

@xirzec @mpodwysocki @witemple-msft You may mainly focus on the changes in openai_modular smoke test which are in
packages/typespec-test/test/openai_modular/generated/typespec-ts folder of this repo.

[xirzec]

This looks correct to me. @mpodwysocki does this follow the same conventions as notification hub?

[xirzec]

Looks good to me! The generated examples are what I expected and this seems to be well aligned with what we have done for NotificationHubs.

[MaryGao]

Overall LGTM, only have one small concern for the inconsistancy import between the api layer and classical layer for models.

#1915 (comment)

[qiaozha]

@bterlson I am going to merge this PR now, feel free to let me know if you found anything, we can always change back 😊