Add rust_ast.nu (#1175)

**Repo link**: https://github.com/graves/nu_rust_ast

**Description**:
A Rust symbol harvesting and analysis tool. It parses Rust source code
into structured Nushell records, enabling deep introspection of
functions, structs, traits, impls, and more. With built-in utilities for
call graph visualization, dependency usage mapping, and colorized symbol
trees, it transforms raw Rust source into actionable, queryable data.

**Why I think it’s awesome**:
- It **unlocks Rust code analysis entirely from the shell**, enabling
you to quickly comprehend large codebases, even when they don't build.
- **Structured Symbol Tables**: Extracts rich metadata for every Rust
item (fqpaths, spans, signatures, visibility, doc comments, bodies).
- **Call Graph Analysis**: Quickly answer "who calls what" and "what
depends on this" with forward and reverse traversals.
- **Dependency Usage Insights**: Map out how external crates are used in
your project with precision.
- **Optimized for Large Projects**: Synthesizes file module nodes,
estimates token counts, and scales to crates of any size.
- **Seamless Nushell Integration**: All outputs are native Nushell
records, perfect for filtering, querying, and piping into other
commands.

It goes beyond static browsing — this tool empowers you to interactively
explore and reason about Rust codebases without needing a buildable
project or IDE.

> Disclaimer: I’m the author of the tool.

Author: graves