move ast command to the debug group (#8077)

# Description

This PR tweaks how `ast` works a tiny bit by outputting values in stead
of eprintln!'s. It also moves the `ast` command into the folder with the
rest of the debug commands and changes the category to debug.

I started adding some tests but couldn't figure out a good way to do it
since every `ast` command contains spans that will be different on each
invocation.

# User-Facing Changes



# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/src/debug/mod.rs

@@ -1,3 +1,4 @@
+mod ast;
 mod debug_;
 mod explain;
 mod inspect;
@@ -10,6 +11,7 @@ mod view_files;
 mod view_source;
 mod view_span;
 
+pub use ast::Ast;
 pub use debug_::Debug;
 pub use explain::Explain;
 pub use inspect::Inspect;