[gin] Add flag for the RegExp Unicode set notation proposal

Note that the proposal has already shipped on the V8 side but we forgot
to add a gin flag for it.

Bug: v8:11935
Change-Id: I506011d1300d32147ac3500fa70c543fb1433531
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4235460
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1103193}

diff --git a/gin/gin_features.h b/gin/gin_features.h
index 6be95b01..34ba3cc 100644
--- a/gin/gin_features.h
+++ b/gin/gin_features.h

@@ -49,6 +49,7 @@
 GIN_EXPORT BASE_DECLARE_FEATURE(kJavaScriptChangeArrayByCopy);
 GIN_EXPORT BASE_DECLARE_FEATURE(kJavaScriptRabGsab);
 GIN_EXPORT BASE_DECLARE_FEATURE(kJavaScriptStringIsWellFormed);
+GIN_EXPORT BASE_DECLARE_FEATURE(kJavaScriptRegExpUnicodeSets);
 GIN_EXPORT BASE_DECLARE_FEATURE(kWebAssemblyTailCall);
 
 }  // namespace features