Add a meta tag to indicate custom elements manifest location on webpage

Documented here: https://github.com/webcomponents/custom-elements-manifest/pull/70

As in some situations, the tooling might not have access to a package.json, it would be beneficial to be able to indicate the location of the custom elements manifest in the html page. A meta tag should be enough for this.

This came up in a discussion with @claviska (https://github.com/claviska), when discussing CEM and DevTools support with the shoelace documentation site.

This would allow documentation sites to indicate the location of the CEM through a meta tag, allowing the devtools to find the manifest for an enhanced experience on the docs page.