Build for evergreen browsers

[TorsteinHonsi]

Implicitly, remove support for IE9-11 in the built JavaScript files.

• Create a legacy path, where all JavaScript files and modules are compiled into /es5/ on code.highcharts.com and npm etc. This legacy path will use the existing, pre-v11 setup. Consider doing this early, in a maintenance time, to give implementers time to switch before v11.
• Early warning that IE11 will stop working with Highcharts v11 unless implementers load the /es5 path. Blog article plus notice on compliance page.
• Now build the main files for evergreen browsers. A possible target for the TypeScript compiler is to allow features that have been supported for the last two years.
• Remove the MSPointer file/logic from the main build, and add it only to es5 builds. This will also serve as a POC for adding future polyfills in the es5 build.
• Update docs. Browser compatibility page especially.