add match guards (#9621)

## description

this pr adds [match
guards](https://doc.rust-lang.org/reference/expressions/match-expr.html#match-guards)
to match patterns
```nushell
match $x {
   _ if $x starts-with 'nu' => {},
   $x => {}
}
```

these work pretty much like rust's match guards, with few limitations:

1. multiple matches using the `|` are not (yet?) supported
 
```nushell
match $num {
    0 | _ if (is-odd $num) => {},
    _ => {}
}
```

2. blocks cannot be used as guards, (yet?)

```nushell
match $num {
    $x if { $x ** $x == inf } => {},
     _ => {}
}
```

## checklist
- [x] syntax
- [x] syntax highlighting[^1]
- [x] semantics
- [x] tests
- [x] clean up

[^1]: defered for another pr

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

@@ -1,12 +1,11 @@
-use serde::{Deserialize, Serialize};
-
-use crate::{Span, VarId};
-
 use super::Expression;
+use crate::{Span, VarId};
+use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct MatchPattern {
     pub pattern: Pattern,
+    pub guard: Option<Expression>,
     pub span: Span,
 }
 

crates/nu-protocol/src/shell_error.rs

@@ -1073,6 +1073,18 @@ pub enum ShellError {
         #[label("This operation was interrupted")]
         span: Option<Span>,
     },
+
+    /// An attempt to use, as a match guard, an expression that
+    /// does not resolve into a boolean
+    #[error("Match guard not bool")]
+    #[diagnostic(
+        code(nu::shell::match_guard_not_bool),
+        help("Match guards should evaluate to a boolean")
+    )]
+    MatchGuardNotBool {
+        #[label("not a boolean expression")]
+        span: Span,
+    },
 }
 
 impl From<std::io::Error> for ShellError {