Be maximally liberal in the interpretation of the browser field

[evanw]

The browser field in package.json is an informal convention followed by browser-specific bundlers that allows package authors to substitute certain node-specific import paths with alternative browser-specific import paths. It doesn't have a rigorous specification and the canonical description of the feature doesn't include any tests. As a result, each bundler implements this feature differently. I have tried to create a survey of how different bundlers interpret the browser field and the results are very inconsistent.

This PR attempts to change esbuild to support the union of the behavior of all other bundlers. That way if people have the browser field working with some other bundler and they switch to esbuild, the browser field shouldn't ever suddenly stop working. This seemed like the most principled approach to take in this situation.

The drawback of this approach is that it means the browser field may start working when switching to esbuild when it was previously not working. This could cause bugs, but I consider this to be a problem with the package (i.e. not using a more well-supported form of the browser field), not a problem with esbuild itself.

Fixes #740