Add src/ directory to the restate-sdk package

[igalshilman]

This PR adds the src/ directory to the list of files to be included in the restate-sdk package.

Running npm pack creates a tarball with the ./src directory included.

Note that, with this change, the bundle size is: 3,1M

[StephanEwen]

looks good in general. how much of that size is generated protobuf code?

[igalshilman]

@StephanEwen The generated protobuf:

572K	./src/generated
1,3M	./dist/generated

This will get smaller, once we move to the new API.

I'll proceed in merging this for now, and if a bundle size will become an issue, we'll find a better way.