Dynamic Import Support and Output Proper ES Modules

[Chad Killingsworth]

As of the 20210505 release, the compiler has full support for dynamic imports even in advanced mode. Details here: https://github.com/google/closure-compiler/wiki/JS-Modules#dynamic-import-expressions

Also supported is an output type of ES Modules. This really impacts code splitting and makes that much easier if you can support only browsers with ES Module support (so pretty much everything if you no longer care about Internet Explorer).  See https://github.com/google/closure-compiler/wiki/Chunk-output-for-dynamic-loading for details.