fix: don't expose incorrect ESM entry in exports

[sodatea]

Fixes #66.

Node.js sees .js files as CJS modules if there's no type: "module" in package.json. So index.esm.js can't be imported from ES modules.

However, CJS modules can be imported from ES modules, named exports are also supported if it is statically analyzable.
So simply removing the import field should be just fine.

The module field is only used by bundlers, so keeping .js extension is harmless.

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A better fix would be using a proper .mjs extension for the ESM build. But I haven't investigated #63, so I chose the safer approach.

[sibbng]

I would suggest naming that import field to browser. So we can keep using it on browser environment. Like jspm.io which is doesn't transform files on the fly like Skypack.