[Cashtab] Patch dependency webpack error for local-ecpair

Summary:
T2730

While the 3-part stack for implementing local ecpair worked under npm start, it did not work in prod. This is because webpack compression removed BigInteger typing. This diff reserves BigInteger typing.

Test Plan:

npm run build
npm i -g serve
serve -s build

Navigate to localhost:3000
Send an XEC tx
Send an eToken tx

This diff is also deployed at https://cashtab-local-dev.netlify.app/
Visit this site on mobile, send an XEC tx, send an eToken tx

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D12638