Issue 4125 - [emscripten] Convert filter classes to C++

compiled/StringScanner.h

+#ifndef ADBLOCK_PLUS_STRING_SCANNER_H
+#define ADBLOCK_PLUS_STRING_SCANNER_H
+
+#include <string>
+
+class StringScanner
+{
+private:
+  const std::u16string str;
+  size_t pos;
+  size_t end;
+public:
+  StringScanner(const std::u16string& str) : str(str), pos(0), end(str.length()) {}
+
+  bool done()
+  {
+    return pos >= end;
+  }
+
+  size_t position()
+  {
+    return pos - 1;
+  }
+
+  char16_t next()
+  {
+    return done() ? u'\0' : str[pos++];
+  }
+};
+
+#endif