Change type of flag defaults to Option<Value> (#9085)

# Description
Follow-up of #8940. As @bobhy pointed out, it makes sense for the
behaviour of flags to match the one for positional arguments, where
default values are of type `Option<Value>` instead of
`Option<Expression>`.

# User-Facing Changes
The same ones from the original PR:
- Flag default values will now be parsed as constants.
- If the default value is not a constant, a parser error is displayed.

# Tests + Formatting

A [new
test](e34e2d35f4/src/tests/test_engine.rs (L338-L344))
has been added to verify the new restriction.

src/tests/test_engine.rs

@@ -328,13 +328,21 @@ fn default_value_constant() -> TestResult {
 }
 
 #[test]
-fn default_value_not_constant() -> TestResult {
+fn default_value_not_constant1() -> TestResult {
     fail_test(
         r#"def foo [x = ("foo" | str length)] { $x }; foo"#,
         "expected a constant",
     )
 }
 
+#[test]
+fn default_value_not_constant2() -> TestResult {
+    fail_test(
+        r#"def foo [--x = ("foo" | str length)] { $x }; foo"#,
+        "expected a constant",
+    )
+}
+
 #[test]
 fn loose_each() -> TestResult {
     run_test(