feat(const): implement run_const for const (#15082)

- fixes #14559

# Description
Allow using `const` keyword in const contexts. The `run_const` body is
basically empty as the actual logic is already handled by the parser.

# User-Facing Changes
`const` keyword can now be used in `const` contexts.

# Tests + Formatting

I'll add some tests before marking this ready. 

- 🟢 toolkit fmt
- 🟢 toolkit clippy
- 🟢 toolkit test
- 🟢 toolkit test stdlib

# After Submitting
N/A

---------

Co-authored-by: Bahex <17417311+Bahex@users.noreply.github.com>

crates/nu-cmd-lang/src/core_commands/const_.rs

@@ -72,6 +72,19 @@ impl Command for Const {
         }
     }
 
+    fn run_const(
+        &self,
+        _working_set: &StateWorkingSet,
+        _call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        Ok(PipelineData::empty())
+    }
+
+    fn is_const(&self) -> bool {
+        true
+    }
+
     fn examples(&self) -> Vec<Example> {
         vec![
             Example {