Chore: Absorb adapt-authoring-defaultplugins into contentplugin

[taylortom]

Fixes #91

Chore

• Absorbed adapt-authoring-defaultplugins functionality into the contentplugin module
• Added isAddedByDefault boolean property directly to the contentplugin schema (replaces the $patch schema from defaultplugins)
• Extracted default plugin logic into lib/utils/addDefaultPlugins.js utility function
• Hooked into content.preInsertHook from ContentPluginModule.init() to auto-add default plugins to new courses
• Added 14 unit tests covering all edge cases

Testing

• Run npm test — all 50 tests pass
• npx standard passes with no warnings

[github-actions]

🎉 This PR is included in version 1.6.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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