Set type to History.state

[pedrobslisboa]

I'm planning to work on this issue so I'm opening it aligned to this point in CONTRIBUTING.md:

Please ensure you are working on an open issue. If necessary, you can
open the issue and then start working. This will give people a chance to
verify the issue and potentially save work.

Description

As the TODO on type state says, it "should be "anything that can be serializable" (MDN REF)

Should we make it type state('a) = Js.t('a);?