relocate debug commands (#8071)

# Description

Now that we've landed the debug commands we were working on, let's
relocate them to an easier place to find all of them. That's what this
PR does.

The only actual code change was changing the `timeit` command to a
`Category::Debug` command. The rest is just moving things around and
hooking them up.

# 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/lib.rs

@@ -4,6 +4,7 @@ mod charting;
 mod conversions;
 mod core_commands;
 mod date;
+mod debug;
 mod default_context;
 mod deprecated;
 mod env;
@@ -34,6 +35,7 @@ pub use charting::*;
 pub use conversions::*;
 pub use core_commands::*;
 pub use date::*;
+pub use debug::*;
 pub use default_context::*;
 pub use deprecated::*;
 pub use env::*;