Move ThirdPartyNpm files into ThirdParty

[ebogo1]

Does what the title says, part of #9473.

The main changes are in the gulpfile and .gitignore, @mramato to make sure I did this right.

[cesium-concierge]

Thanks for the pull request @ebogo1!

• ✔️ Signed CLA found.
• ❔ 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.
• Works (or fails gracefully) in IE11.

[mramato]

At a high level glance, this looks fine.. we'll need to compare the full npm staging branch to main to be sure before merging the whole thing for release.

[lilleyse]

Merging into the staging branch where we'll do a final pass over everything