refactor: define HTML document interfaces locally

[jderochervlk]

Tracking issue: #342

Summary

• add the public HTML module for shared HTML option types
• define document-structure and content-element records in their owning modules
• add the missing HTMLUnknownElement interface
• update those modules to use their local interface types

Temporary state

• the matching aggregate HTML definitions remain in DOM/DomTypes while later HTML families are migrated
• refactor: remove DOM compatibility types #310 removes the aggregate copies and the DomTypes compatibility module
• the new modules remain in the broad DOM source folder until the follow-up Option 5 stack establishes the HTML folder feature

Review focus

• ownership of shared options in HTML
• completeness and public compatibility of the document/content element interfaces

Verification

• npm run build
• npm test
• npm run format:check

[jderochervlk]

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