Enable back button functionality, clear url on load

[WardBrian]

See discussion before merge in #84

This makes two changes:

1. Query parameters are cleared on load, rather than on first edit. This "godbolt.org style" is my preference in the design space, but it also technically made item 2 much easier:
2. Clicking the back button into a history item with the query parameters present leads to them actually loading in the UI

This also made it so the reducer no longer needs a callback to clear the URL on edit, since it will be done during load.

[magland]

I still prefer keeping the url intact as long as possible, and I'd like to propose a solution to this in the future. But because this is a simplification, I would be in favor of merging at this point.

[jsoules]

Approved, pending a recommended change to simplify the initial-load useEffect hook in the context provider.

[jsoules]

It occurs to me that this check might not be necessary if we remove the stan-playground-saved-state key from local storage regardless of which branch we take in the conditional.

Suppose searchParams is nontrivial. Then the first branch of the conditional below will trigger, ensuring that they'll be 0 on any subsequent loop.

Otherwise we'll check the status of the localStorage key. If that is set, we'll load it; if it is unset, no-op.

So, either we loaded the local storage (and don't need it any more), or we loaded the query string (and are actively ignoring the local storage values). Either way, the hook no longer needs to depend on the data member, which simplifies it further.

[WardBrian]

Nice spot!