Change type of parameter default values to Option<Value> (#8940)

# Description

Fixes #8939.

# User-Facing Changes

- Parameter default values will now be parsed as constants.
- If the default value is not a constant, a parser error is displayed.

# Tests + Formatting

The [only affected
test](d42c2b2dbc/src/tests/test_engine.rs (L325-L328))
has been updated to reflect the new behavior.

crates/nu-parser/src/parser.rs

@@ -3735,8 +3735,19 @@ pub fn parse_signature_helper(working_set: &mut StateWorkingSet, span: Span) ->
                                                 }
                                             }
                                         }
+
+                                        *default_value = if let Ok(constant) =
+                                            eval_constant(working_set, &expression)
+                                        {
+                                            Some(constant)
+                                        } else {
+                                            working_set.error(ParseError::NonConstantDefaultValue(
+                                                expression.span,
+                                            ));
+                                            None
+                                        };
+
                                         *shape = expression.ty.to_shape();
-                                        *default_value = Some(expression);
                                         *required = false;
                                     }
                                     Arg::RestPositional(..) => {