Add startsWith function to IString (#1393)

Author: aheejin

src/emscripten-optimizer/istring.h

@@ -136,6 +136,15 @@ struct IString {
 
   bool is() const     { return str != nullptr; }
   bool isNull() const { return str == nullptr; }
+
+  bool startsWith(const char *prefix) const {
+    const char *ptr = str;
+    while (true) {
+      if (*prefix == 0) return true;
+      if (*ptr == 0) return false;
+      if (*ptr++ != *prefix++) return false;
+    }
+  }
 };
 
 } // namespace cashew