[Cashtab] Ensure browserify is installed before extension build

Summary:
T2322

For npm run extension to work properly, browserify must be installed globally. Currently, if the user runs npm run extension without browserify installed globally, some files will be corrupted.

This diff adds a preextension script, so that npm i -g browserify will always be run before npm run extension. This prevents a user from unintentionally building the extension without browserify installed.

Test Plan:

npm uninstall -g browserify
npm run extension

Observe that browserify is installed before the extension script is run. Confirm by checking git status and noting that no files are changed.

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11232