Add domain sharding to js runtime

[benjervis]

In order to support domain sharding for HTTP1.1 users, we need to apply shards in two places. Firstly, in the generated HTML, which is done at request time by the SSR server (as we need to know whether this is a 1.1 request or not). Secondly, when requesting bundles from within other bundles we need to shard those requests as well, and not just rely on the parent shard.

To support this, I've made some changes to the JS Runtime, adding in a sharded version of helpers/bundle-url which is activated if the domain sharding config is provided in a build.

"@atlaspack/runtime-js": {
  "domainSharding": {
    "maxShards": 5,
    "cookieName": "should.shard.domains"
  }
}

bundle-url-shards started out as a copy of bundle-url, but has been refactored a bit to make the more complicated logic easier to work with. That number of functions also became a little tricky to track, so the file is now a TypeScript file which will be compiled in the client build.

There was a todo in here to migrate getOrigin to just using the URL constructor once we didn't need to support IE 11 anymore, which we don't, so I've made that change and brought it across to bundle-url also.

Lastly there are a bunch of tests to make sure the sharding logic does what it's meant to, and that the runtime generates the call with the right arguments.

[JakeLane]

nit: a conventional for loop would be faster

[mattcompiles]

Nice one bud!