core: Loaders to es6 classes

[DefinitelyMaybe]

supported by #19982 i.e. managed to build without errors after implementing those changes.

Did run npm test but we will see what comes back from the automated tests. I'm feeling slightly weary of the current testing, future me problem.

note:
Many loaders have the following pattern:

class NewLoader extends Loader {
  // ...
  load( url, onLoad, onProgress, onError ) {

    const scope = this;
    // ...
    const loader = new FileLoader( this.manager );
    loader.setPath( this.path );
    loader.setRequestHeader( this.requestHeader );
    loader.load( url, function ( text ) {
    // ...
    }, onProgress, onError);
  }
}

It doesn't look very good to me however instead of trying to tackle that within the same PR I'll follow up on this one in a future PR.

[DefinitelyMaybe]

as #19982 has been merged, I can either add example loaders to this PR or see if I can get the tests to re-run. might need to do a dummy commit but we'll see.

[DefinitelyMaybe]

removed changes to loaders which were used by any example script.

[DefinitelyMaybe]

conflicts resolved

[mrdoob]

Thanks!