CesiumJS CI is Migrating from Travis to GithHub Actions

Hi all,

Cesium as a whole is migrating away from Travis CI to GitHub Actions for continuous integration (CI).

For the CesiumJS repo itself, this will minorly change the UI for checks that run as part of pull requests. More information is available in the updated CesiumJS Build Guide section on CI.

GitHub actions are enabled by default in all repos. If you have a CesiumJS fork which makes use of CI,

Please let us know if you have any questions!