Updates for integration with lit-ssr-hacker-news app

[kevinpschaaf]

• Makes dom-shim a factory to return a unique window object per call to importModule
• Adds node-fetch shim for fetch to dom-shim, along with a couple other required globals for it
• Adds support to importModule for importing built-in node modules in VM context (required for e.g. node-fetch)
• Ensures identifier for VM modules is unique per VM context (important to avoid caching issues with dynamic imports)
• Adds Koa middleware for rendering static HTML files through render-lit-html
  • Scripts to be loaded on server should be marked with ssr attribute
  • Scripts to be loaded only on server (e.g. for fetching/initializing data) can be marked with type="ssr-only" to prevent loading on client
  • If a module script loaded on the server exports an initializeSSR method, it will be invoked & awaited
  • If initializeSSR returns any values, they will be interpolated into <!--lit-ssr-value--> markers placed in the HTML file

Associated PR's:

• https://github.com/PolymerLabs/lit-ssr-hacker-news/pull/1
• Adds SSR support for unsafeHTML/SVG lit/lit#1179