nu-cmd-lang cleanup (#12609)

# Description
This PR does miscellaneous cleanup in some of the commands from
`nu-cmd-lang`.

# User-Facing Changes
None.

# After Submitting
Cleanup the other commands in `nu-cmd-lang`.

crates/nu-protocol/src/ast/expression.rs

@@ -1,5 +1,5 @@
 use crate::{
-    ast::{Argument, Block, Expr, ExternalArgument, ImportPattern, RecordItem},
+    ast::{Argument, Block, Expr, ExternalArgument, ImportPattern, MatchPattern, RecordItem},
     engine::StateWorkingSet,
     BlockId, DeclId, Signature, Span, Type, VarId, IN_VARIABLE_ID,
 };
@@ -79,6 +79,13 @@ impl Expression {
         }
     }
 
+    pub fn as_match_block(&self) -> Option<&[(MatchPattern, Expression)]> {
+        match &self.expr {
+            Expr::MatchBlock(matches) => Some(matches),
+            _ => None,
+        }
+    }
+
     pub fn as_signature(&self) -> Option<Box<Signature>> {
         match &self.expr {
             Expr::Signature(sig) => Some(sig.clone()),