Implement atob and btoa methods for worker

Moved the Atob and Btoa methods ouside the impl for WorkerMethod trait and made them publicly accessible from WorkerGlobalScopeMethods via proxy methods.

The webidl file already contained the `implements` line and all I did there was uncomment the line with `WorkerGlobalScope implements WindowBase64`. I don't know if that does it but running `make -f makefile.cargo` worked just fine and `mach build` did not complain about method declaration not being present in the trait definition so I believe it worked fine.

Fixes issue #3238