Include Custom Element Manifest for Shadow Root Processors

[JRJurman]

Summary

Right now intellisense when using the Tram-Lite custom attributes can be lacking. However, there is a standard for custom-element manifests, see https://github.com/webcomponents/custom-elements-manifest

This provides auto-completion and hover-over information, similar to what's already available for native HTML elements. You can see interactive examples of this here: https://github.com/microsoft/vscode-custom-data/tree/main/samples/webcomponents

We should investigate what might be possible here, if we can suggest / expose some of the attributes used in the shadowRootProcessors.

[JRJurman]

Frustratingly, it looks like Custom Element manifests are only for custom-elements, so documenting custom attributes on existing elements isn't possible 🤔