[Feature] Generate Source Map when generating typescript code

[wu-s-john]

Problem description

I am currently using fern now for my company. We want to run code coverage against our Fern client APIs to see if exercise if we covered all the endpoints. Namely, there are some issues that we don't know what we are catching. Adding this for fern would be really useful.

Why would it be useful?

Namely, this is very useful for a code quality point of very. If I have a high degree of code coverage, then that means I would have a good master of what test cases need to be covered.

Describe the solution (optional)

When we generate the code, it just generates the .ts files. It would be good to generate the .js files as well

Additional context

Add any other context or screenshots.