make the ansi command const (#11682)

# Description

This PR changes the `ansi` command to be a `const` command. 

- ~~It's breaking because I found that I had to change the way `ansi` is
used in scripts a little bit.
https://github.com/nushell/nu_scripts/pull/751~~

- I had to change one of the examples because apparently `const` can't
be tested yet.

- ~~I'm not sure this is right at all
https://github.com/nushell/nushell/pull/11682/files#diff-ba932369a40eb40d6e1985eac1c784af403dab4500a7f0568e593900bf6cd740R654-R655.
I just didn't want to duplicate a ton of code. Maybe if I duplicated the
code it wouldn't be a breaking change because it would have a run and
run_const?~~

- I had to add `opt_const` to CallExt.

/cc @kubouch Can you take a look at this? I'm a little iffy if I'm doing
this right, or even if we should do this at all.

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crates/nu-engine/src/call_ext.rs

@@ -1,6 +1,7 @@
 use nu_protocol::{
     ast::Call,
-    engine::{EngineState, Stack},
+    engine::{EngineState, Stack, StateWorkingSet},
+    eval_const::eval_constant,
     FromValue, ShellError, Value,
 };
 
@@ -36,6 +37,12 @@ pub trait CallExt {
         pos: usize,
     ) -> Result<Option<T>, ShellError>;
 
+    fn opt_const<T: FromValue>(
+        &self,
+        working_set: &StateWorkingSet,
+        pos: usize,
+    ) -> Result<Option<T>, ShellError>;
+
     fn req<T: FromValue>(
         &self,
         engine_state: &EngineState,
@@ -126,6 +133,19 @@ impl CallExt for Call {
         }
     }
 
+    fn opt_const<T: FromValue>(
+        &self,
+        working_set: &StateWorkingSet,
+        pos: usize,
+    ) -> Result<Option<T>, ShellError> {
+        if let Some(expr) = self.positional_nth(pos) {
+            let result = eval_constant(working_set, expr)?;
+            FromValue::from_value(result).map(Some)
+        } else {
+            Ok(None)
+        }
+    }
+
     fn req<T: FromValue>(
         &self,
         engine_state: &EngineState,