Do not extend `window` with all of the exports in UMD bundle, rather just a property of `window`

[flevi29]

In our current Rollup setup, we're extending window with all of the exports as is. This is a bad practice, and can introduce property name conflicts. In #1740 Vite doesn't even support extending just the bare window, it only allows us to specify the property name on which it will insert all of the exports (I had to introduce a hacky footer code to allow for this deprecation instead of a breaking change).