Use @microsoft/api-extractor

[stevengum]

Current integration testing has proven insufficient in avoiding breaking changes, and we've had cases were files were not properly exported. Using api-extractor can help with avoiding these scenarios:

https://www.npmjs.com/package/@microsoft/api-extractor

https://api-extractor.com/

[stevengum]

I will be disabling tsdoc warnings generated from @microsoft/api-extractor for now, will reenable on case-by-case basis in the future in coordination with the SDK and Docs teams.

Edit: The P0 is avoiding breaking changes.

[stevengum]

#1962 is not enabling API checks on botframework-connector due to microsoft/rushstack#1029 which may be fixed by microsoft/rushstack#1796.

Modifying the generated code by hand would be rather time consuming, and botframework-connector is a package that doesn't undergo much delta, which is why I'm not enabling the API check on this lib at this time.