Update code to stop detecting flask

[hugomrdias]

Hello snap devs! For launch it is very important that you update your dapps to stop detecting MetaMask Flask. Instead, you should detect for the existence of the Snaps API so your dapp is ready to work with the stable release. This will work with Flask and with the stable extension.

We have updated our documentation with a guide on connecting to a snap from your dapp: https://docs.metamask.io/snaps/how-to/work-with-existing-snaps/

However, you may encounter issues when a user is running multiple wallets in the same browser (such as Phantom and Coinbase Wallet). I have published example code for working around these issues here: https://github.com/Montoya/snap-connect-test

Please reach out if you have questions or issues with your dapps.