Fix v8::External::Value/New for the new ExternalPointerTypeTag API

[gdavidkov]

Problem

V8 added an ExternalPointerTypeTag parameter to v8::External::New and v8::External::Value and removed the prior overloads:

static Local<External> New(Isolate*, void*, ExternalPointerTypeTag);
void* Value(ExternalPointerTypeTag) const;

The new signatures are gated by V8's external-pointer-tagging build configuration, signaled by the public macro V8_EXTERNAL_POINTER_TAG_COUNT. A V8_MAJOR_VERSION check is unreliable here because Node and Chromium ship divergent V8 builds — the feature can be enabled in Electron's V8 and not in upstream Node's V8 at the same nominal major version.

nan still calls the removed forms — 28 sites in nan_callbacks_12_inl.h (the imp::*CallbackWrapper trampolines) and 3 in nan_implementation_12_inl.h (the External / Function / FunctionTemplate factories). Both are header-inline, so any addon that includes <nan.h> fails to build against a tag-enabled V8. Reproduced against Electron 42.0.0-beta.8 (Node 24.15.0) on Windows + MSVC 2022:

nan_callbacks_12_inl.h(189): error C2664:
  cannot convert from 'v8::Isolate *' to 'v8::ExternalPointerTypeTag'
nan_implementation_12_inl.h(79): error C2660:
  'v8::External::New' does not take 2 arguments

Fix

Two private inline helpers in the existing imp:: namespace, one per file next to its callers:

inline void* GetExternalPointer(v8::Local<v8::External> ext);
inline v8::Local<v8::External> NewExternal(v8::Isolate*, void*);

Each selects the tag-taking signature when V8_EXTERNAL_POINTER_TAG_COUNT is defined, passing v8::kExternalPointerTypeTagDefault (V8's no-tagging default for embedders that do not partition externals by type), and falls back to the legacy form otherwise. All 31 callsites are routed through the helpers. Both use the same tag, so create/read tags match by construction.

Backwards compatibility

Every change is gated on #ifdef V8_EXTERNAL_POINTER_TAG_COUNT. The #else branches reproduce the prior code byte-for-byte — no-op on every V8 build that doesn't define the macro.

Tests

test/cpp/news.cpp::NewExternal and test/cpp/nannew.cpp::testExternal round-trip a void* through Nan::New<v8::External> and read it back via raw v8::External::Value(). They hit the same compile break as nan's headers; the same gate is applied at each test site. NewExternal exercises imp::NewExternal directly; imp::GetExternalPointer is exercised every time any NAN_METHOD test runs from JS, since imp::FunctionCallbackWrapper unpacks the registered callback through it before dispatch.

Out of scope

V8 14 work already landed (#1010, #1013); this PR addresses only the External API change. nan_callbacks_pre_12_inl.h / nan_implementation_pre_12_inl.h target Node ≤ 0.12 (pre-V8-3.x), which uses a still-older single-arg form, so a tag-enabled V8 never selects those files.

Verified

Builds clean against Electron 42.0.0-beta.8 (Node 24.15.0) on Windows + MSVC 2022. CI will exercise both branches across the existing matrix.