Use ES6 modules under Node versions that support them.
Use ES6 modules under Node versions that support them.

When running in a version of Node that supports ES6, this will now load
the ES6 version of Cesium instead of the transpiled modules or modules
loaded via ESM. It does it by setting the module type and entry points in
package.json.

There are 3 use cases this allows:

1. Roll-up and other tools that support resolving front-end dependencies
via node_modules can now make use of Cesium.
`import { defined, JulianDate } from 'cesium';` can now be used in such
systems instead of specifying the relative path to Source/Cesium.js This
makes it easier to write reusable components that rely on Cesium.

2. Since we are using Cesium ES6, node bundlers can perform treeshaking
and eliminate unused code, basically allow for building combined minified
node code with tools like roll-up.

3. Requiring in individual modules, such as `cesium/Source/Core/Cartesian3.js`
is now possible in situations where pulling the primary entry point is
not desired.

The one downside of this change is that ES6 versions of Node will no
longer automatically pull in the combined/minimized version of Cesium.js,
though its still technically possible to do via manual esm usage. This
may (or may not) impact performance under node in certain situations but
ultimately a build process is properly the preferred method for optimizing
node applications.

likangning93/cesium,CesiumGS/cesium,YonatanKra/cesium,YonatanKra/cesium,AnalyticalGraphicsInc/cesium,likangning93/cesium,likangning93/cesium,CesiumGS/cesium,likangning93/cesium,progsung/cesium,progsung/cesium,CesiumGS/cesium,CesiumGS/cesium,YonatanKra/cesium,AnalyticalGraphicsInc/cesium,YonatanKra/cesium,likangning93/cesium,CesiumGS/cesium