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.h

@@ -61,6 +61,9 @@ public:
     T const& current_token();
     void reconsume_current_input_token();
 
+    int position() const { return m_iterator_offset; }
+    void rewind_to_position(int);
+
     void skip_whitespace();
 
     void dump_all_tokens();