Demo playground import maps, validate playground shortcodes, allow custom editor/preview heights

[aomarks]

• Demonstrate how to import lit@next in playgrounds by setting project.json importMap.imports to skypack URLs (fixes Playgrounds can resolve to lit@next #113).

  lit@next doesn't actually work on skypack right now because of an error relating to missing hydrate-support.js: https://cdn.skypack.dev/new/lit-element@v3.0.0-pre.2/dist=es2020

• Validate playground project shortcodes. Shows errors during 11ty build from invalid project.json path, or any missing files (cc @arthurevans, fixes Show error when a file can't be loaded #133).

• Add ability to customize editor and preview height in project.json (cc @sorvell, fixes Ability to control height of playground example editor and preview #136).

  {
    files: { ... },
    previewHeight: "250px"
  }

[github-actions]

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https://pr138-aebc5ee---lit-dev-bvxw3ycs6q-uw.a.run.app/
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