parser: implement while loops, port while test

Implement in parser.c:
- parseWhileExpr: while (cond) body, with optional payload,
  continue expression, and else clause
- while_simple, while_cont, while AST nodes

Port test "while else err prong with no block".

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

parser_test.zig

@@ -1517,6 +1517,18 @@ test "zig fmt: whitespace fixes" {
     );
 }
 
+test "zig fmt: while else err prong with no block" {
+    try testCanonical(
+        \\test "" {
+        \\    const result = while (returnError()) |value| {
+        \\        break value;
+        \\    } else |err| @as(i32, 2);
+        \\    try expect(result == 2);
+        \\}
+        \\
+    );
+}
+
 test "zig fmt: tagged union with enum values" {
     try testCanonical(
         \\const MultipleChoice2 = union(enum(u32)) {