feat: custom polyfil imports for node

[relu91]

I'm migrating an old project from browserify to esbuild and I found out that I can't override the default polyfils with some custom implementation. I'm not sure if this is not a planned feature, but, given the fact that the implementation seems straightforward, I've added the required changes.

The use case that I have is that I need to override the default readable-stream implementation with the latest release, but it might be helpful also for others.

Futher infos

• the browserify build script that I'm porting is hosted here.
• I've also opened an issue in jspm-core: https://github.com/jspm/jspm-core/issues/95