Serve from a separate build folder during development

[ggetz]

Fixes #10836

Rather than having the npm start task build and serve content from Build/CesiumUnminified, this builds and copies content to Build/CesiumDev. This ensures we can still have a fresh build when running the task, while not overriding or removing any content from a previous build (such as make-zip).

In order to keep the existing Build/CesiumUnminified paths working without re-writing, server.jsnow redirects requests for Build/CesiumUnminified to the fresh Build/CesiumDev build.

[cesium-concierge]

Thanks for the pull request @ggetz!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[ggetz]

@sanjeetsuhag Could you please review?

[sanjeetsuhag]

Thanks @ggetz! Looks good. Can confirm that this fixes the issue with the release workflow.