Flowistry like feature

Flowistry is a very cool and useful tool that gray out codes that are not related to the code under cursor. Now that we have a functioning MIR, we can implement a similar feature in rust-analyzer. The flowistry paper explains how it works, and it's internal analysis is published as a crate but I don't think we can reuse that much since it is heavily dependent on the rustc internals. A challenge in reimplementing that in r-a is that we ignore lifetime annotations currently in type lowering, but I think it wouldn't create problems for start, we can assume all lifetimes are static at cost of some false positive gray areas. Another challenge is constructing a ast to mir map, for finding the mir node under cursor.

But why we should implement it in rust-analyzer when flowistry already exists? Implementing it in rust-analyzer has some benefits:

• Flowistry focus mode will be disabled if you change the code, limiting its usability.
• It's start time is a bit slow.
• It needs nightly, which doubles the target folder size, and has other nightly problems.
• It only supports vscode, but r-a is cross editor.
• It's discoverability is a little lower than r-a.

cc @willcrichton

Start with the Flowistry paper and its published analysis crate to understand the requested behavior. Then inspect rust-analyzer's existing MIR and type-lowering work, focusing on the stated challenge of mapping AST locations to MIR nodes. Done would require a scoped design and an implemented cross-editor equivalent of Flowistry's focus mode.