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.

src/tests/test_engine.rs

@@ -323,8 +323,16 @@ fn default_value12() -> TestResult {
 }
 
 #[test]
-fn default_value_expression() -> TestResult {
-    run_test(r#"def foo [x = ("foo" | str length)] { $x }; foo"#, "3")
+fn default_value_constant() -> TestResult {
+    run_test(r#"def foo [x = "foo"] { $x }; foo"#, "foo")
+}
+
+#[test]
+fn default_value_not_constant() -> TestResult {
+    fail_test(
+        r#"def foo [x = ("foo" | str length)] { $x }; foo"#,
+        "expected a constant",
+    )
 }
 
 #[test]