Customise shim per entrypoint

[halvardssm]

Hi! I have been using this package for a little while, and stumbled now upon a slight issue. When creating a multi-entrypoint package where each entrypoint supports a different subset of features, for example:

• one entrypoint for system, where fs and env is needed
• one entrypoint for browser, where document is needed
• one entrypoint for react components etc

It would be great if each entrypoint (the interface EntryPoint) would contain a shims property (interface ShimOptions) that would override the top-level shims property.

[halvardssm]

@dsherret I could potentially assist with a PR here, but could you give me some pointers on where you think this change should happen? I had a look through the codebase, and it seems a bit like a larger refactor would be needed, so if you have any pointers, that could save me a lot of time :)