[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