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/default_context.rs

@@ -29,7 +29,6 @@ pub fn create_default_context() -> EngineState {
         // Core
         bind_command! {
             Alias,
-            Ast,
             Break,
             Commandline,
             Const,
@@ -178,6 +177,7 @@ pub fn create_default_context() -> EngineState {
 
         // Debug
         bind_command! {
+            Ast,
             Debug,
             Explain,
             Inspect,