Export an interface-only module so it's easier to dev with

[ArjunBhuptani]

When importing Connext modules, you are forced to import 60+ contracts. We should create a repo to export only the core contracts needed for integration

[just-a-node]

Proposal

Create a separate repo as connext/interfaces which will be the main repo to install with Forge and npm

• copy subset of contract interfaces relevant for integrators (IConnext.sol, IXReceiver.sol, any others? that might be it)
• create abstract contract implementing IXReceiver that includes onlySource authentication modifier
• [later] create abstract contract implementing IXReceiver that includes rescue/orphan functionality
• set up CI to publish npm package

[rhlsthrm]

This sounds good to me!

[ArjunBhuptani]

Looks good, should also potentially include a Mock.sol (talk to Derby Finance team as they have already written this)

[just-a-node]

Closing this out as @connext/interfaces v1.0.0 is set up now.

Extracting abstract patterns into #3638 and mocks into #3640.