Use `requestIdleCallback` when polling for wallets, if available

This is a follow on from #327. If we're going to poll for wallets indefinitely in `scopePollingDetectionStrategy` (eg. to capture the case where a wallet is installed in the browser mid-session) then let's use `requestIdleCallback` wherever available. This will make it so that the polling work never blocks work that the app has scheduled at a high priority.

See https://developer.mozilla.org/en-US/docs/Web/API/Background_Tasks_API for how this works.