Use Custom Elements Manifest to warn on deprecated attrs

[bennypowers]

The Custom Elements Manifest lets us document which attributes (css parts, methods, events, etc) are deprecated on our custom elements.

It would be swell if selector-finder would read the manifest for a page (perhaps based on the version referenced in the page importmap), then output warnings for usage of deprecated attributes.

for example, rh-cta's color-palette attribute is deprecated in the 1.4 series