Add contenteditable adapter packages

[developer-1px]

Closes #188.

Summary

• add @interactive-os/json-document-contenteditable-web as a DOM contenteditable adapter for text surfaces
• add @interactive-os/json-document-contenteditable-react as the thin React timing wrapper
• register both packages as official extensions and source aliases

Verification

• npm run extensions:verify
• npm run typecheck -w @interactive-os/json-document
• npm test -w @interactive-os/json-document -- text-surface
• git diff --cached --check

Stacked on #187 / issue-186-text-surface.