Remove Value::MatchPattern (#11356)

# Description
`Value::MatchPattern` implies that `MatchPattern`s are first-class
values. This PR removes this case, and commands must now instead use
`Expr::MatchPattern` to extract `MatchPattern`s just like how the
`match` command does using `Expr::MatchBlock`.

# User-Facing Changes
Breaking API change for `nu_protocol` crate.

crates/nu-cmd-lang/src/example_support.rs

@@ -278,9 +278,6 @@ impl<'a> std::fmt::Debug for DebuggableValue<'a> {
                 let rec = val.collect().map_err(|_| std::fmt::Error)?;
                 write!(f, "LazyRecord({:?})", DebuggableValue(&rec))
             }
-            Value::MatchPattern { val, .. } => {
-                write!(f, "MatchPattern({:?})", val)
-            }
         }
     }
 }