Remove unused pipe node definitions

The `Pipe` node and its associated logic have been removed from the AST.
This commit cleans up the code by removing all references to this
defunct node in the analyzer, binder, captures, dumper, lowering,
macros, optimizer, and type checker. The parser no longer recognizes the
`pipe` keyword.

src/ast/compiler/binder.rs

@@ -358,22 +358,6 @@ impl Binder {
                 }
             }
 
-            SyntaxKind::Pipe { inputs, lambda } => {
-                let mut bound_inputs = Vec::with_capacity(inputs.len());
-                for input in inputs {
-                    bound_inputs.push(Rc::new(self.bind(input, ExprContext::Expression, diag)));
-                }
-                let bound_lambda =
-                    Rc::new(self.bind(lambda.as_ref(), ExprContext::Expression, diag));
-                self.make_node(
-                    node.identity.clone(),
-                    NodeKind::Pipe {
-                        inputs: bound_inputs,
-                        lambda: bound_lambda,
-                    },
-                )
-            }
-
             SyntaxKind::Lambda { params, body, .. } => {
                 let identity = node.identity.clone();
                 self.functions