parser: implement switch, port switch comment tests

Implement parseSwitchExpr in parser.c:
- switch(expr) { cases... } with case items, ranges, else
- switch_case_one and switch_case node types
- Proper scratch management for nested case items

Port tests:
- "switch comment before prong"
- "switch comment after prong"

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

parser_test.zig

@@ -2137,6 +2137,31 @@ test "zig fmt: function call with multiline argument" {
     );
 }
 
+test "zig fmt: switch comment before prong" {
+    try testCanonical(
+        \\comptime {
+        \\    switch (a) {
+        \\        // hi
+        \\        0 => {},
+        \\    }
+        \\}
+        \\
+    );
+}
+
+test "zig fmt: switch comment after prong" {
+    try testCanonical(
+        \\comptime {
+        \\    switch (a) {
+        \\        0,
+        \\        // hi
+        \\        => {},
+        \\    }
+        \\}
+        \\
+    );
+}
+
 test "zig fmt: if-else with comment before else" {
     try testCanonical(
         \\comptime {