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-color-config/src/style_computer.rs

@@ -131,8 +131,7 @@ impl<'a> StyleComputer<'a> {
             Value::Closure { .. }
             | Value::CustomValue { .. }
             | Value::Error { .. }
-            | Value::LazyRecord { .. }
-            | Value::MatchPattern { .. } => TextStyle::basic_left(),
+            | Value::LazyRecord { .. } => TextStyle::basic_left(),
         }
     }