DX: Fix `defined` and `Check` types for CesiumJS devs

[jjspace]

Description

• Add defined.d.ts which will now get picked up and provide the correct type predicates and assertions so JS Type checking will understand things are defined after this call
  • Originally added in https://github.com/CesiumGS/cesium/pull/11455 which is fantastic for external contributors but local development doesn't look at the generated index.d.ts file for types like it will with the new defined.d.ts file

Before	After

• Fixed Check.d.ts for local development so the TS server understands it's a default export not an exported object
  • This was originally added in https://github.com/CesiumGS/cesium/pull/11901 to help external users of CesiumJS, primarily the ion team

Before	After

• Updated the build process to account for these files and changes. The built Cesium.d.ts and engine/index.d.ts files should be identical to the previous ones.

Issue number and link

No issue, just a small DX issue I've grown too tired of seeing

Testing plan

• In main run npm run built-ts
• Create a copy of Source/Cesium.d.ts and packages/engine/index.d.ts somewhere you can check later
• Switch to this branch and run npm run build-ts
• Do a diff of those files and ensure they are identical. This confirms our external use will be the same downstream
• To test locally find a usage of defined and Check locally
• Turn on the VSCode setting Check JS for type checking with the TS server (This may or may not be necessary, I'm not sure)
• Verify that in main the types aren't correct as shown above, verify that in this branch they are
• Turn off CheckJS if you're not ready to see a lot of other (false positive) errors scattered in our code

Author checklist

• [ ] I have submitted a Contributor License Agreement
• [ ] I have added my name to CONTRIBUTORS.md
• [ ] I have updated CHANGES.md with a short summary of my change
• [ ] I have added or updated unit tests to ensure consistent code coverage
• [ ] I have updated the inline documentation, and included code examples where relevant
• [ ] I have performed a self-review of my code

[github-actions[bot]]

Thank you for the pull request, @jjspace!

:white_check_mark: We can confirm we have a CLA on file for you.

[jjspace]

@javagl Would you mind taking a look at this? I think you've been looking into our type system and typescript adjacent stuff recently so you might be a good person to provide input.

[jjspace]

After enabling the "Check JS" functionality in VS Code, it did complain about many things. But it did not generate the "...is possibly undefined" messages for me.

@javagl I sometimes find I have to run the Restart TS Server or Reload Window commands to refresh typescript. They seem to cache some things internally or something that makes it get out of sync, especially in JS environments that aren't actually TS

This is NOT in the scope of this PR, but I cannot look at the gulpfile.js without pointing out that this is one of my pet peeves

I fully agree with this sentiment, I'd love if the logic was centralized because I always forget to change both any time I touch this. Definitely outside the scope of this PR though. I'm happy to review a PR for cleaning up the gulpfile but as you say, every time I touch this file I'm worried about breaking something