[Cashtab] Check if user has browser extension installed

Summary:
T3283

If new monitorExtension flag is set, check if user has Cashtab browser extension installed. Print this info to the console.

Note: will build this out behind a feature flag as there are a few things we want to handle before we start showing popups to our users (see task).

Test Plan:
npm test

Change monitorExtension to true
npm start and view dev console. Should print your extension installation status.
Disable the extension or install the extension and refresh the page --> see opposite status

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D14478