Add Pyodide handles

[hoodmane]

@ryanking13 ran into the problem in pyodide/pyodide#3184 (comment) that it's inconvenient to send results from one @run_in_pyodide invocation to another. This adds a mechanism to do so. A SeleniumHandle (perhaps can be named better) can be returned to the host and when passed back into another @run_in_pyodide function one can use it to access the object.

This should allow us to make fixtures specifically for use with @run_in_pyodide functions that return one or more SeleniumHandles.

[ryanking13]

Wow, thanks @hoodmane! I didn't look at the details yet but it looks very cool. I'll review this during weekends, or if it is not available, next Monday.

[ryanking13]

@koenvo might want to review this too, as it looks like #46 has some code conflict with this.

[koenvo]

I can change my PR when this one is merged. The provision_id is not used in my PR. Shouldn't conflict.

[rth]

Thanks! It looks quite interesting, and overall LGTM. Though I'll let @ryanking13 do a second review.

Could you please add a changelog entry and an example to the readme on how to use this?

[ryanking13]

Thanks a lot @hoodmane! I have some minor comments, otherwise looks good to me.

[ryanking13]

Thanks again! Please update the changelog and add some examples in README if possible as Roman mentioned, then feel free to merge. I think we can release this with the tag 0.23.0.