Add a Custom Elements Manifest

[jamesnw]

It would be useful for users to add a Custom Elements Manifest (CEM) - https://github.com/webcomponents/custom-elements-manifest.

[devnook]

Do we have some examples of use cases where this manifest would be used? I'm a bit hesitant to add this (as a maintenance overhead) unless it is really needed by someone.

[jamesnw]

The CEM is similar to a .d.ts file, in that it gives tooling information about the custom element for contextual help, autocomplete, linting, etc. For instance, it could generate VS Code Custom Data using CEM Tools.

I'm uncertain of what best practice is for distributed web components, but I'd propose including both the CEM and the VS Code Custom Data output, and potentially adding others as consumers request it.