[catalog-server] Bootstrapping packages

[justinfagnani]

We need to have the catalog do an initial search / crawl of packages with custom elements manifests. We can try to find such packages in a few ways:

1. Include a hard-coded list of packages that we know have custom elements
2. Search npm for keywords like "web components", "custom elements", etc via the search endpoint.
3. Scrape the list of packages that depend on well-known web component libraries like Lit, Stencil, Polymer, FAST, LWC, etc. There doesn't appear to be a registry API for this.