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.

crates/nu-protocol/src/signature.rs

@@ -2,7 +2,6 @@ use serde::Deserialize;
 use serde::Serialize;
 
 use crate::ast::Call;
-use crate::ast::Expression;
 use crate::engine::Command;
 use crate::engine::EngineState;
 use crate::engine::Stack;
@@ -25,7 +24,7 @@ pub struct Flag {
 
     // For custom commands
     pub var_id: Option<VarId>,
-    pub default_value: Option<Expression>,
+    pub default_value: Option<Value>,
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]