Add command_prelude module (#12291)

# Description
When implementing a `Command`, one must also import all the types
present in the function signatures for `Command`. This makes it so that
we often import the same set of types in each command implementation
file. E.g., something like this:
```rust
use nu_protocol::ast::Call;
use nu_protocol::engine::{Command, EngineState, Stack};
use nu_protocol::{
    record, Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData,
    ShellError, Signature, Span, Type, Value,
};
```

This PR adds the `nu_engine::command_prelude` module which contains the
necessary and commonly used types to implement a `Command`:
```rust
// command_prelude.rs
pub use crate::CallExt;
pub use nu_protocol::{
    ast::{Call, CellPath},
    engine::{Command, EngineState, Stack},
    record, Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, IntoSpanned,
    PipelineData, Record, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
};
```

This should reduce the boilerplate needed to implement a command and
also gives us a place to track the breadth of the `Command` API. I tried
to be conservative with what went into the prelude modules, since it
might be hard/annoying to remove items from the prelude in the future.
Let me know if something should be included or excluded.

crates/nu-command/src/system/run_external.rs

@@ -1,25 +1,18 @@
 use nu_cmd_base::hook::eval_hook;
-use nu_engine::env_to_strings;
-use nu_engine::get_eval_expression;
-use nu_engine::CallExt;
-use nu_protocol::{
-    ast::{Call, Expr},
-    did_you_mean,
-    engine::{Command, EngineState, Stack},
-    Category, Example, IntoSpanned, IoStream, ListStream, NuGlob, PipelineData, RawStream,
-    ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
-};
+use nu_engine::{command_prelude::*, env_to_strings, get_eval_expression};
+use nu_protocol::{ast::Expr, did_you_mean, IoStream, ListStream, NuGlob, RawStream};
 use nu_system::ForegroundChild;
 use nu_utils::IgnoreCaseExt;
 use os_pipe::PipeReader;
 use pathdiff::diff_paths;
-use std::collections::HashMap;
-use std::io::{BufRead, BufReader, Read, Write};
-use std::path::{Path, PathBuf};
-use std::process::{Command as CommandSys, Stdio};
-use std::sync::mpsc;
-use std::sync::Arc;
-use std::thread;
+use std::{
+    collections::HashMap,
+    io::{BufRead, BufReader, Read, Write},
+    path::{Path, PathBuf},
+    process::{Command as CommandSys, Stdio},
+    sync::{mpsc, Arc},
+    thread,
+};
 
 #[derive(Clone)]
 pub struct External;