TODO: Extract import map into a generated JSON file.

[MicahZoltu]

Currently, the vendoring script mutates index.html. Instead, index.html should have

<script type='importmap' srcp='./generated/import-map.json'></script>

in it and we should generate import-map.json when running the vendoring script. This way we don't have committed code in the repository.

Only reason this is an issue instead of a PR is because I don't have the time right now to verify this change would work in all browsers and doesn't fail for some reason. If someone wants to do that, would be happy to accept a PR!