Warn users about running multiple MetaMask (Q4)

Having multiple MM running is not recommended.

Options:

- [ ] Put a note in the flask onboarding page
- [ ] Use the extension messaging API and the immutable, platform-dependent extension IDs to communicate with other versions of MetaMask to detect and display a warning to the user
- We can use the extension-provider as a reference implementation for how to use the messaging API:
https://github.com/MetaMask/extension-provider
- We should store the IDs as enums somewhere. The IDs can be retrieved by installing the main extension and/or Flask from the extension store of a particular browser and just check what the ID is. The ID of every extension in the Chrome and Firefox stores are unique, permanent, and set by the browser vendors.

Start with the Flask onboarding page and the extension-provider reference implementation linked in the issue. Review the extension messaging API and the platform-specific extension IDs for Chrome and Firefox. The work is done when users are warned about multiple MetaMask versions, with the relevant IDs stored as enums.

Written by the indexing model from the issue text.