Add browser shim for External.js.

[CXuesong]

Fixes #96, hopefully.

[DaanVandenBosch]

Awesome, @JWally, do you think we can get this merged?

[qspark-zz]

Thank you. Confirmed; fixes the following errors in angular:

Module not found: Error: Can't resolve 'fs' in ...
Module not found: Error: Can't resolve 'child_process' in ...

[raffclar]

This fixes lots of issues with esbuild and webpack. For example projects that use d3-dag will encounter Critical dependency: the request of a dependency is an expression on build due to this dependency.

[Hazelfire]

Agreed, would love this to be merged please!