Fix: Implement ~ and ~@ syntax to prevent parser deadlock

Adds parsing for the `~` (placeholder) and `~@` (splice) syntax.

- `~expr` parses as a placeholder node.
- `~@expr` parses as a splice node.

src/ast/parser.rs

@@ -378,6 +378,25 @@ impl<'a> Parser<'a> {
                     ty: (),
                 }
             }
+            TokenKind::Tilde => {
+                let token = self.advance(); // consume ~
+                if *self.peek() == TokenKind::At {
+                    self.advance(); // consume @
+                    let expr = self.parse_expression();
+                    Node {
+                        identity: NodeIdentity::new(token.location),
+                        kind: UntypedKind::Splice(Box::new(expr)),
+                        ty: (),
+                    }
+                } else {
+                    let expr = self.parse_expression();
+                    Node {
+                        identity: NodeIdentity::new(token.location),
+                        kind: UntypedKind::Placeholder(Box::new(expr)),
+                        ty: (),
+                    }
+                }
+            }
             _ => {
                 self.diagnostics.push_error(
                     format!(
@@ -386,6 +405,7 @@ impl<'a> Parser<'a> {
                     ),
                     Some(NodeIdentity::new(self.current_token.location)),
                 );
+                self.advance();
                 Node {
                     identity: NodeIdentity::new(self.current_token.location),
                     kind: UntypedKind::Error,