refactor: Add TypeScript types for the Editors' Redux state

[bradenmacdonald]

Description

This PR starts to add type information to the messy Redux state used by the editors, mostly focusing on the state shared by all editors and the problem editor.

Supporting information

There should be no user-visible changes / no functionality changes.

Testing instructions

Test the problem editor and make sure there are no regressions.

Other information

Private ref: MNG-4479

[openedx-webhooks]

Thanks for the pull request, @bradenmacdonald!

What's next?

Please work through the following steps to get your changes ready for engineering review:

🔘 Get product approval

If you haven't already, check this list to see if your contribution needs to go through the product review process.

• If it does, you'll need to submit a product proposal for your contribution, and have it reviewed by the Product Working Group.
  • This process (including the steps you'll need to take) is documented here.
• If it doesn't, simply proceed with the next step.

🔘 Provide context

To help your reviewers and other members of the community understand the purpose and larger context of your changes, feel free to add as much of the following information to the PR description as you can:

• Dependencies

  This PR must be merged before / after / at the same time as ...

• Blockers

  This PR is waiting for OEP-1234 to be accepted.

• Timeline information

  This PR must be merged by XX date because ...

• Partner information

  This is for a course on edx.org.

• Supporting documentation
• Relevant Open edX discussion forum threads

🔘 Get a green build

If one or more checks are failing, continue working on your changes until this is no longer the case and your build turns green.

🔘 Let us know that your PR is ready for review:

Who will review my changes?

This repository is currently maintained by @openedx/2u-tnl. Tag them in a comment and let them know that your changes are ready for review.

Where can I find more information?

If you'd like to get more details on all aspects of the review process for open source pull requests (OSPRs), check out the following resources:

• Overview of Review Process for Community Contributions
• Pull Request Status Guide
• Making changes to your pull request

When can I expect my changes to be merged?

Our goal is to get community contributions seen and reviewed as efficiently as possible.

However, the amount of time that it takes to review and merge a PR can vary significantly based on factors such as:

• The size and impact of the changes that it introduces
• The need for product review
• Maintenance status of the parent repository

💡 As a result it may take up to several weeks or months to complete a review and merge your PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.92%. Comparing base (abe68ac) to head (64b7260).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1537      +/-   ##
==========================================
+ Coverage   92.91%   92.92%   +0.01%     
==========================================
  Files        1066     1068       +2     
  Lines       20988    21029      +41     
  Branches     4538     4469      -69     
==========================================
+ Hits        19501    19542      +41     
- Misses       1414     1421       +7     
+ Partials       73       66       -7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ChrisChV]

Looks good 👍

• I tested this: I tested the problem editor
• I read through the code and considered the security, stability and performance implications of the changes.
• I tested that the UI can be used with a keyboard only (tab order, keyboard controls).
• Includes tests for bugfixes and/or features added.