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-command/src/debug/explain.rs

@@ -259,6 +259,5 @@ pub fn debug_string_without_formatting(value: &Value) -> String {
         Value::Binary { val, .. } => format!("{val:?}"),
         Value::CellPath { val, .. } => val.to_string(),
         Value::CustomValue { val, .. } => val.value_string(),
-        Value::MatchPattern { val, .. } => format!("{:?}", val),
     }
 }