feat(material/core): add experimental theme demo

[andrewseguin]

Preview: https://ng-dev-previews-comp--pr-angular-components-29636-dev-6fy759rh.web.app/theme

Be sure to enable M3 and System Theme in the header

[github-actions[bot]]

Deployed dev-app for 4a90895987e3d354088f67489f13225858f4e3d3 to: https://ng-dev-previews-comp--pr-angular-components-29636-dev-6fy759rh.web.app

Note: As new commits are pushed to this pull request, this link is updated after the preview is rebuilt.

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