Continuation of #62976.

These contribution points should be similar to the user configurations:

{
  "html.experimental.custom.tags": [
    "./web-components.json",
    "./element-details/web-components.json"
  ],
  "html.experimental.custom.attributes": [
    "./web-component-attributes.json"
  ]
}

They allow extension authors to publish a list of tags / attributes together with its frameworks, and VS Code could load them for HTML language features.