New Rule: Private Variables

[justinkaseman]

Description

Add a solhint rule to enforce the following recommendation from the Chainlink contracts style guide:

All contract variables should be private by default. Getters should be explicitly written and documented when you want to expose a variable publicly.
Whether a getter function reads from storage, a constant, or calculates a value from somewhere else, that’s all implementation details that should not be exposed to the consumer by casing or other conventions.

https://github.com/smartcontractkit/chainlink/blob/3a74dc13b2d42316803b26b2254caa12f7beccb4/contracts/STYLE_GUIDE.md?plain=1#L323

Steps

1. Make a solhint rule and open a PR against this repository's master branch
2. Once merged, open a PR to the Chainlink repository that updates the STYLE_GUIDE.md to reference the new rule