Add SSR support

[emdap]

This moves the scrollbar width measurement into an effect so that it won't run during SSR.

To test this out, I linked my locally built react-custom-scroller to a repo that has prerendering enabled, and verified that there were no errors while running or building.

[treycucco]

This looks like a good change Emma. I want to have Vitor look over it since I haven't done anything in this repo. My main suggestions, listed in the comments below is this:

Don't unconditionally call set state, use the cached value being defined to bail out. Default the state to use the cached value, or fallback if necessary. And wrap all that logic (the state and the effect) into a named hook that only returns the width, so all the hooks dealing with reading and calculating the width are batched up together.

[linear-code]

GRO-677

[buzinas]

Overall looks good, but I'm wondering if we could avoid the effect by just letting the calculation happens as it did before.