Add ability to mutate page props

[vinnygambiny]

This PR allows mutating the page props with the usePage function.

This is for these situations https://github.com/inertiajs/inertia/discussions/78

You have only to do usePage().props.valueName = 'newValue'

[punyflash]

I don't think that should be a thing. Data which loaded the page should be accessible all the time, no matter what. In case you need it to be reactive and changeable - just create a custom reference that was initialized with the prop value.

[innocenzi]

@punyflash there are definitely situations where this would be handy (eg. infinite loading), and Inertia should be more flexible about this.

[pedroborges]

I'm closing this PR since the main use case discussed here and on #78 is supporting infinite scrolling. That's already coming to v2.0 soon, as you can see in Taylor's Laracon keynote 🚀