Cratification: Break out nu_cmd_lang into a separate crate (#8181)

# Description

This breaks out the core_commands into a separate crate called
nu_cmd_lang

_(Thank you for improving Nushell. Please, check our [contributing
guide](../CONTRIBUTING.md) and talk to the core team before making major
changes.)_

_(Description of your pull request goes here. **Provide examples and/or
screenshots** if your changes affect the user experience.)_

# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking 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/dataframe/test_dataframe.rs

@@ -8,7 +8,7 @@ use nu_protocol::{
 use super::eager::ToDataFrame;
 use super::expressions::ExprCol;
 use super::lazy::{LazyCollect, ToLazyFrame};
-use crate::Let;
+use nu_cmd_lang::Let;
 
 pub fn test_dataframe(cmds: Vec<Box<dyn Command + 'static>>) {
     if cmds.is_empty() {