Migrate Backlogs to JavaScript classes, type improvements

[myabc]

⚠️ This PR is based off of #19429. Please review/merge that PR first.

Ticket

https://community.openproject.org/wp/66575

What are you trying to accomplish?

Migrates backlogs from Crockford Objects to JavaScript classes.

Screenshots

What approach did you choose and why?

Merge checklist

• [ ] Added/updated tests
• [ ] Added/updated documentation in Lookbook (patterns, previews, etc)
• [ ] Tested major browsers (Chrome, Firefox, Edge, ...)

[myabc]

@copilot please go through this draft conversion of a very old part of the codebase from "Crockford Objects"/custom Object factories (RB.Object.initialize) to modern JavaScript/TypeScript classes. Look for any possible errors in the conversion and correct please.

You may want to start with the suggestions that you provided as comments in this Pull Request: https://github.com/opf/openproject/pull/19907#discussion_r2496474788

Do the following before committing or pushing:

npm run generate-typings
npm run build
npm run test

Although there are no test cases that cover these particular files, this will at least check TypeScript compiles.

[Copilot]

@myabc I've opened a new pull request, #20945, to work on those changes. Once the pull request is ready, I'll request review from you.