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/strings/encode_decode/base64.rs

@@ -1,12 +1,18 @@
 use base64::{
-    alphabet, engine::general_purpose::NO_PAD, engine::general_purpose::PAD,
-    engine::GeneralPurpose, Engine,
+    alphabet,
+    engine::{
+        general_purpose::{NO_PAD, PAD},
+        GeneralPurpose,
+    },
+    Engine,
 };
 use nu_cmd_base::input_handler::{operate as general_operate, CmdArgument};
 use nu_engine::CallExt;
-use nu_protocol::ast::{Call, CellPath};
-use nu_protocol::engine::{EngineState, Stack};
-use nu_protocol::{PipelineData, ShellError, Span, Spanned, Value};
+use nu_protocol::{
+    ast::{Call, CellPath},
+    engine::{EngineState, Stack},
+    PipelineData, ShellError, Span, Spanned, Value,
+};
 
 pub const CHARACTER_SET_DESC: &str = "specify the character rules for encoding the input.\n\
                     \tValid values are 'standard', 'standard-no-padding', 'url-safe', 'url-safe-no-padding',\

crates/nu-command/src/strings/encode_decode/decode.rs

@@ -1,10 +1,4 @@
-use nu_engine::CallExt;
-use nu_protocol::ast::Call;
-use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{
-    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Spanned,
-    SyntaxShape, Type, Value,
-};
+use nu_engine::command_prelude::*;
 
 #[derive(Clone)]
 pub struct Decode;

crates/nu-command/src/strings/encode_decode/decode_base64.rs

@@ -1,9 +1,5 @@
 use super::base64::{operate, ActionType, CHARACTER_SET_DESC};
-use nu_protocol::ast::Call;
-use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
-};
+use nu_engine::command_prelude::*;
 
 #[derive(Clone)]
 pub struct DecodeBase64;

crates/nu-command/src/strings/encode_decode/encode.rs

@@ -1,10 +1,4 @@
-use nu_engine::CallExt;
-use nu_protocol::ast::Call;
-use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{
-    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Spanned,
-    SyntaxShape, Type, Value,
-};
+use nu_engine::command_prelude::*;
 
 #[derive(Clone)]
 pub struct Encode;

crates/nu-command/src/strings/encode_decode/encode_base64.rs

@@ -1,9 +1,5 @@
 use super::base64::{operate, ActionType, CHARACTER_SET_DESC};
-use nu_protocol::ast::Call;
-use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Type, Value,
-};
+use nu_engine::command_prelude::*;
 
 #[derive(Clone)]
 pub struct EncodeBase64;