Fix local developer experience

[vgrichina]

Looks like our tooling is currently broken for local developer experience, at the very least config.js doesn't provide proper keyPath (see https://github.com/nearprotocol/nearcore/pull/2885#discussion_r444577043 for details).

Checklist:

• [ ] nearlib fixed keyPath
• [ ] near-shell fixed keyPath
• [ ] create-near-app fixed keyPath
• [ ] examples fixed keyPath
• [ ] creating account has fallback for local development
• [ ] near login has fallback for local development
• [ ] create-near-app templates have "wallet" fallback for local development which work both for login and signing arbitrary transactions
• [ ] examples have "wallet" fallback for local development which work both for login and signing arbitrary transactions
• [ ] there is obvious and easy way to start local node with separate network, e.g. nearup local
• [ ] launching node for local development and other caveats documented at https://docs.near.org/docs/local-setup/local-dev-node

[chadoh]

While fixing this, we might also consider moving default config.js values into near-api-js

This would allow apps to stop including a src/config.js file

near-api-js would need to allow overriding specific values with environment variables

This relates to the idea I outlined in https://github.com/nearprotocol/NEPs/pull/31#discussion_r437799000