Add support for Flowistry

[kellytk]

From https://github.com/willcrichton/flowistry:

Flowistry is a tool that analyzes the information flow of Rust programs. Flowistry understands whether it's possible for one piece of code to affect another. Flowistry integrates into the IDE to provide a "focus mode" which helps you focus on the code that's related to your current task.

The animated gif helpfully illustrates its functionality.