improve compatibility with web bundlers

[aduh95]

• With Rollup, it works just fine with the following config:

  import { nodeResolve } from "@rollup/plugin-node-resolve";
  import OMT from "@surma/rollup-plugin-off-main-thread";
  import url from "@rollup/plugin-url";
  
  export default {
    input: "src/index.js",
    output: {
      dir: "output",
      format: "amd",
    },
    plugins: [
      nodeResolve(),
      OMT(),
      url({
        include: "**/*.wasm",
      }),
    ],
  };

• With Webpack, you need to tweak one or two lines in the code – documented in the comments in this PR.

• With Parcel, it should just work without any plugin once WebWorkers: Support new Worker( new URL() ) syntax parcel-bundler/parcel#5430 is resolved.

Fixes: #17