refactor(filesystem): prefer our fs wrapper to node's with lint rule

[Hweinstock]

Problem

We want to avoid future unnecessary use of fs module and maximize usage of our wrapper fs. Benefits include:

• more control over interfacing with fs.
• increased web compatibility.
• minimizing dependencies.

Solution

• add a lint rule to discourage future imports of fs.
• leave current uses as exceptions.
• Planning follow up PR to migrate some of the easy uses of fs to our wrapper.

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License: I confirm that my contribution is made under the terms of the Apache 2.0 license.