TerrainPicker: ES6 + type checking

[donmccurdy]

Description

Updates TerrainPicker.js to ES6, and enables type checking. No functional changes, but catches a few minor typos in JSDoc.

Issue number and link

• Let us slowly move to Typescript. (instructions inside) #4434
• Use ES6 "class" as module syntax #8359

Testing plan

Diff on Cesium.d.ts (./scripts/diff-types.sh) is empty. Unit tests should pass.

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

AI acknowledgment

• I used AI to generate content in this PR
• If yes, I have reviewed the AI-generated content before submitting

If yes, I used the following Tools(s) and/or Service(s):

If yes, I used the following Model(s):

[github-actions]

Thank you for the pull request, @donmccurdy!

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