feat: added items command for Records (#8640)

# Description

This PR adds an `items` command which allows the user to iterate over
both `columns` and `values` of a `Record<>` type at the same time.


![image](https://user-images.githubusercontent.com/3835355/227976277-c9badbb2-2e31-4243-8d00-7e28f2289587.png)

# User-Facing Changes

No breaking changes, only a new `items` command.

# Formatting

- `cargo fmt --all -- --check` 👌 
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` 👌
- `cargo test --workspace` 👌

crates/nu-command/src/default_context.rs

@@ -56,6 +56,7 @@ pub fn create_default_context() -> EngineState {
             GroupBy,
             Headers,
             Insert,
+            Items,
             Join,
             SplitBy,
             Take,