Move JS SDKs into a monorepo

[morleyzhi]

Is your feature request related to a problem? Please describe. It's annoying to toggle between three different codebases, each with independent prettier / eslint / build settings. It would be a lot easier to manage all three packages from one repo.

Describe the solution you'd like

• [ ] init lerna
• [ ] Move stellar-sdk files into packages dir
• [ ] Move js-xdr and js-stellar-base here
• [ ] Update CI and deploy settings

Describe alternatives you've considered We don't technically have to do this (which is why it hasn't happened yet), but the sooner we do, the more we can reap the small conveniences.

[bartekn]

This is fine except I think js-xdr should stay in it's own repo. It can be useful in other projects and committers shouldn't have to pull all Stellar-related code. However, I don't have very strong feelings about it, it just seems more clean.