LibWeb: Add TokenStream::rewind_to_position()

Parsing media queries sometimes requires significant back-tracking, so
`reconsume_current_input_token()` was not good enough.
`rewind_to_position()` lets you reconsume an erbitrary number of tokens
to return to an earlier point in the stream, which you previously saved
from `TokenStream::position()`.

Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -106,6 +106,13 @@ void TokenStream<T>::reconsume_current_input_token()
         --m_iterator_offset;
 }
 
+template<typename T>
+void TokenStream<T>::rewind_to_position(int position)
+{
+    VERIFY(position <= m_iterator_offset);
+    m_iterator_offset = position;
+}
+
 template<typename T>
 void TokenStream<T>::skip_whitespace()
 {