Support importing as ESM in Node

[MattiasBuelens]

This uses conditional exports to define the CommonJS and ES module paths for each variant. This enables modern Node versions to load the .mjs files, instead of the CommonJS .js builds.

As explained in the documentation, this will prevent consumers from using anything that is not defined by exports, so they will no longer be to do e.g. require("web-streams-polyfill/dist/polyfill.js"). This is likely a breaking change.

Moreover, consumers may already be using import streams from "web-streams-polyfill" with modern Node, which would give them the CommonJS module as a default export. This will also break, since the default export will be replaced by named exports. Consumers will need to change their code to either import { ReadableStream } ... or import * as streams ... instead.

To do:

• Add tests for loading the different variants as CommonJS or ES modules.

[MattiasBuelens]

This probably needs to wait for microsoft/TypeScript#33079, since the current version of TypeScript won't be able to find the correct type definitions for each subpath export.

[MattiasBuelens]

Superseded by #83.