Custom command input/output types (#9690)

# Description

This adds input/output types to custom commands. These are input/output
pairs that related an input type to an output type.

For example (a single int-to-int input/output pair):

```
def foo []: int -> int { ... }
```

You can also have multiple input/output pairs:
```
def bar []: [int -> string, string -> list<string>] { ... }
```

These types are checked during definition time in the parser. If the
block does not match the type, the user will get a parser error.

This `:` to begin the input/output signatures should immediately follow
the argument signature as shown above.

The PR also improves type parsing by re-using the shape parser. The
shape parser is now the canonical way to parse types/shapes in user
code.

This PR also splits `extern` into `extern`/`extern-wrapped` because of
the parser limitation that a multi-span argument (which Signature now
is) can't precede an optional argument. `extern-wrapped` now takes the
required block that was previously optional.

# User-Facing Changes

The change to `extern` to split into `extern` and `extern-wrapped` is a
breaking change.

# Tests + Formatting
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> **Note**
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crates/nu-cmd-lang/src/core_commands/extern_.rs

@@ -19,7 +19,6 @@ impl Command for Extern {
             .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("def_name", SyntaxShape::String, "definition name")
             .required("params", SyntaxShape::Signature, "parameters")
-            .optional("body", SyntaxShape::Block, "wrapper function block")
             .category(Category::Core)
     }
 

crates/nu-cmd-lang/src/core_commands/extern_wrapped.rs

@@ -0,0 +1,52 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Type};
+
+#[derive(Clone)]
+pub struct ExternWrapped;
+
+impl Command for ExternWrapped {
+    fn name(&self) -> &str {
+        "extern-wrapped"
+    }
+
+    fn usage(&self) -> &str {
+        "Define a signature for an external command."
+    }
+
+    fn signature(&self) -> nu_protocol::Signature {
+        Signature::build("extern-wrapped")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
+            .required("def_name", SyntaxShape::String, "definition name")
+            .required("params", SyntaxShape::Signature, "parameters")
+            .required("body", SyntaxShape::Block, "wrapper function block")
+            .category(Category::Core)
+    }
+
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nu.html"#
+    }
+
+    fn is_parser_keyword(&self) -> bool {
+        true
+    }
+
+    fn run(
+        &self,
+        _engine_state: &EngineState,
+        _stack: &mut Stack,
+        _call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        Ok(PipelineData::empty())
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Write a signature for an external command",
+            example: r#"extern-wrapped echo [text: string] { print $text }"#,
+            result: None,
+        }]
+    }
+}

crates/nu-cmd-lang/src/core_commands/mod.rs

@@ -17,6 +17,7 @@ mod export_extern;
 mod export_module;
 mod export_use;
 mod extern_;
+mod extern_wrapped;
 mod for_;
 mod hide;
 mod hide_env;
@@ -55,6 +56,7 @@ pub use export_extern::ExportExtern;
 pub use export_module::ExportModule;
 pub use export_use::ExportUse;
 pub use extern_::Extern;
+pub use extern_wrapped::ExternWrapped;
 pub use for_::For;
 pub use hide::Hide;
 pub use hide_env::HideEnv;

crates/nu-cmd-lang/src/default_context.rs

@@ -35,6 +35,7 @@ pub fn create_default_context() -> EngineState {
             ExportUse,
             ExportModule,
             Extern,
+            ExternWrapped,
             For,
             Hide,
             HideEnv,