LibWeb: Suppress false-positive LSan report during WebWorker teardown

Problem: For several weeks now, CI Sanitizer runs have been logging
leaked RustCompiledRegex objects in the WebWorker process when a worker
is torn down while an off-thread script compile is still in flight.

Cause: compile_off_thread() (introduced in 4a7dc45b3f) parses and
compiles a fetched top-level script on a ThreadPool worker. The pool's a
process-lifetime singleton whose workers are never joined before exit —
and that leads to LSan reporting a false positive during teardown.

Fix: Suppress the false positive via __lsan_default_suppressions().

AK/LsanSuppressions.h

@@ -16,7 +16,8 @@ char const* __lsan_default_suppressions()
     // Both Skia and Chromium suppress false positive FontConfig leaks
     // https://github.com/google/skia/blob/main/tools/LsanSuppressions.cpp#L20
     // https://chromium.googlesource.com/chromium/src/build/+/master/sanitizers/lsan_suppressions.cc#25
-    return "leak:FcPatternObjectInsertElt";
+    return "leak:FcPatternObjectInsertElt\n"
+           "leak:rust_compile_regex";
 }
 }
 #endif