[PR] Exporting default types properly so it matches the javascript file.

[LuchoTurtle]

fixes #411

The problem was related to how the default export was being treated from Typescript to Javascript. They didn't match, so what happened is that, after transpiling the Typescript to Javascript, the js files would have a different way of exporting the default AWS variable than what was described in the Typescript files. Read https://github.com/arethetypeswrong/arethetypeswrong.github.io/blob/main/docs/problems/FalseExportDefault.md for a comprehensive overview.

This PR fixes this and changes the way AWS is exported to export = AWS (instead of export default AWS).

In addition to this, the bundle configuration from tsup is now explicit.

[LuchoTurtle]

@nelsonic take a look and approve when possible. I've tested the output bundle (the bundled .js files) in the project https://github.com/aannoune/aws-sdk-mock-issue411 and it's working now.

After approval, I'll publish and merge.

[nelsonic]

@LuchoTurtle code/updates look good.