[v8] Propagate v8_cppgc_microtask_queue flag to common.gypi (#259)

v8_cppgc_microtask_queue affects the public ABI of v8::MicrotaskQueue.
When enabled, it alters the class hierarchy (inheriting from cppgc::GarbageCollected)
and changes the return type of MicrotaskQueue::New().

This flag must be propagated to GYP to ensure that native addons built via
node-gyp are compiled with the identical V8 memory layout and method signatures
as the Node.js executable, preventing silent ABI breakages and segfaults.

TAG=agy
CONV=21b9994c-2327-48b5-b81f-aa460e16a410

common.gypi

diff --git a/common.gypi b/common.gypi
index 36b4b11..b8b3ef6 100644
--- a/common.gypi
+++ b/common.gypi
@@ -87,6 +87,7 @@
     'v8_enable_v8_checks%': 0,
     'v8_use_perfetto': 0,
     'tsan%': 0,
+    'v8_cppgc_microtask_queue%': 0,
 
     ##### end V8 defaults #####
 
@@ -544,6 +545,9 @@
       ['tsan == 1', {
         'defines': ['V8_IS_TSAN',],
       }],
+      ['v8_cppgc_microtask_queue == 1', {
+        'defines': ['V8_CPPGC_MICROTASK_QUEUE',],
+      }],
       ['OS == "win"', {
         'defines': [
           'WIN32',