[Cashtab] Webpack upgrade step 7, add scripts to package.json

Summary:
T1682

Step 7, add npm scripts

These scripts are not expected to work yet. This diff is only following the tested/scripted approach described in T1682.

Test Plan: Review package.json and confirm the added scripts line up with step 7 of T1682

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11247