[ecash-wallet] Init ecash-wallet in monorepo

Summary:
Now that we have type stability with chronik-client and sophisticated tx construction with ecash-lib, we can (and should) create an abstracted typed wallet class to make tx construction easier and more consistent for builders

Some requirements

• Support XEC sends
• Support token methods
• Support agora
• Support "chained" txs, i.e. token sends to more than max outputs, or XEC sends to outputs that would bump tx size above the broadcast limit
• regtest integration
• Should be able to drop this into Cashtab and pull out all of Cashtab's custom tx methods
• HD wallet support
• Construct from WIF

This diff just inits the repo and CI. Next diff will add send XEC method and get that tested with regtest integration. Then will add other methods.

Test Plan:
npm test

I am building this lib incrementally following a personal-use class I have been testing / dogfooding for the last couple of months.

So, any comments on structure, layout, questions about why some things are being initialized this way --- also important.

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Subscribers: tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D17773