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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crates/nu-command/tests/commands/def.rs

@@ -158,8 +158,9 @@ fn def_with_paren_params() {
 
 #[test]
 fn extern_with_block() {
-    let actual =
-        nu!("extern foo [...rest] { print ($rest | str join ',' ) }; foo --bar baz -- -q -u -x");
+    let actual = nu!(
+        "extern-wrapped foo [...rest] { print ($rest | str join ',' ) }; foo --bar baz -- -q -u -x"
+    );
 
     assert_eq!(actual.out, "--bar,baz,--,-q,-u,-x");
 }