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_plugin_gstat/src/gstat.rs

@@ -1,8 +1,6 @@
 use git2::{Branch, BranchType, DescribeOptions, Repository};
 use nu_protocol::{record, IntoSpanned, LabeledError, Span, Spanned, Value};
-use std::fmt::Write;
-use std::ops::BitAnd;
-use std::path::Path;
+use std::{fmt::Write, ops::BitAnd, path::Path};
 
 // git status
 // https://github.com/git/git/blob/9875c515535860450bafd1a177f64f0a478900fa/Documentation/git-status.txt