Converting vislib to use ES6 syntax

[ppisljar]

preparation for future work on vislib

[spalger]

LGTM, tests are passing and the functionality here hasn't changed at all

[Bargs]

Could we put the commit that converts functions to classes in a separate PR? There are some unresolved arguments about whether we should even recommend classes in our styleguide, so I'd like to resolve those before we start converting everything to class. The rest of the changes look great though so I don't want the class argument to block the rest.

[spalger]

This change is only a small part of the much larger #8174. In that pr, several new¹ classes are being defined to replace the classes seen here.

In my eyes, that means these changes are helping to build a comprehensible progression to the final, not a refactor or starting the "converting everything to class" movement. Merging since the "rest of the changes look great".

1: It's pretty obvious that we are fine with new development using class