[Cashtab] Webpack upgrade for extension script

Summary:
T1682

Add npm module needed to build the extension.

Test Plan:

npm ci
npm run extension

Confirm extension builds with no errors [Note: feature testing for extension is not part of this diff]

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11270