[landing] Factor out logic for monitoring WalletConnect modal

Summary:
As part of the WalletConnect 2 update, RainbowKit is now using WalletConnect's Web3Modal.

This modal changes sizes more frequently, and as a result later diffs will complicate the code for monitoring the WalletConnect modal. Ahead of that, I'd like to factor out that logic into a separate hook.

Depends on D8250

Test Plan:
Tested with the rest of the stack. I tested all three SIWE endpoints:

1. Web
2. Native (old flow)
3. Native (new flow)

For each, I tested both Rainbow and WalletConnect (both of which need WalletConnect), and navigated around the WalletConnect modal to various places.

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8251