Add loading attribute for HTMLIframeElement

[1000ch]

• Add a description of loading attribuets for <iframe> element.
• Use 'eager' | 'lazy' instead of string as a type for loading attributes

[github-actions[bot]]

Thanks for the PR!

This section of the codebase is owned by @saschanaz - if they write a comment saying "LGTM" then it will be merged.