Fix V8 external API usage for Electron 42 (#1475)

Electron 42 ships V8 14.8, where v8::External::New and v8::External::Value require an ExternalPointerTypeTag. This breaks native rebuilds with Electron 42 headers.

Add small compatibility macros for v8::External creation/access, following the existing V8 version-guard pattern used for prior Electron compatibility fixes such as #1459. Also pass nullptr to SetNativeDataProperty to avoid ambiguous overload resolution for a missing setter.

Verified with npm test and an Electron 42.1.0 rebuild from source.