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 Make TS definition of defined use type predicate #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 Add TS types for Check, remove circular dependency #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]

Thank you for the pull request, @jjspace!

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

[jjspace]

As per my comment when I did the same conversion into a .d.ts file for Check, I would love if we could skip this replace and pass the types file into tsd-jsdoc but we cannot. I think defined and Check are some of the only types we really need to override to take advantage of stuff like type predicates but if we add more we should definitely revisit this setup as it will quickly become too unwieldy.