js: Export variables and classes for Workspace Matrix extension

As mentioned here the port to ESM (!1499 (merged)) broke the Workspace Matrix extension (and maybe others) because many variables that were previously var are now const that are not exported. This MR exports the variables and classes required for Workspace Matrix.

I also propose to take this one step further by exporting everything again, that was importable before !1499 (merged). This will fix potential issues with other extensions wanting to support GNOME Shell 45. I'll be happy to do this if I get an ok from a maintainer.

Another step further could be to export everything by default to make it easier for extension developers to change the default behavior. But this feels like a rather drastic step somehow.