Generate ESM boilerplate

[marklundin]

This PR adds the ability to conditionally generate ESM based class boilerplate when using createScript(). If the provided filename contains the .mjs suffix it will generate code based on the following boilerplate;

export class ClassName extends pc.ScriptType {

    static name = 'Script Name';

    initialize() {

    }

    update() {

    }

    // uncomment the swap method to enable hot-reloading for this script
    // update the method body to copy state from the old instance
    // swap(old) { };
}

// learn more about scripting here:
// https://developer.playcanvas.com/user-manual/scripting/

[Maksims]

Please add dt into update method for the consistency with the old script template.

[marklundin]

Ahh of course. good catch

[marklundin]

This has been added now in #68