Add per-problem Cargo.toml and subdir structure for Rust LSP support

[Antonio-Bennett]

Summary

• For lang = 'rust', generates subdir crates (e.g., code/1-two_sum/src/lib.rs, Cargo.toml with prob-1-two_sum package, tests.dat).
• Enables full rust-analyzer LSP in editors (autocomplete, diagnostics) without breaking LeetCode API submissions (still extracts code snippet).
• New config: [code] enable_rust_crates = true (default; set false for flat files).
• Backward compatible: Legacy configs load with default true; notes existing flat files.
• Enhancements: Sanitized names, commented deps in Cargo.toml, error handling.

Changes

• Updated paths in helper.rs (conditional subdirs).
• Added dir/Cargo.toml creation in cmds/edit.rs.
• Configurable via new field in config/code.rs.
• Docs: New README section on Rust setup.

Testing

• Manual: leetcode edit 1 creates structure; test 1/exec 1 work.
• LSP: Open dir in Helix/VS Code—full features.
• Flat fallback: Set enable_rust_crates = false and edit another problem.

Closes #204 . Ready for review!

[clearloop]

If I'm not mistaken, rust LSP supports single file, see https://github.com/rust-lang/rust-analyzer/pull/8955, may I ask which editor (+rustc version) are you using?

[Antonio-Bennett]

Sorry @clearloop was on vacation! I use Helix Editor. I guess this is something I need to look into, I couldn't get it to work nicely before