LibWeb/CSS: Expose consumed tokens from TokenStream

In some situations we want to examine a sequence of tokens that have
been already consumed from a TokenStream. These methods let us do so:
- current_index() provides the current internal token index
- tokens_since() provides a span from the given index to the current one

Libraries/LibWeb/CSS/Parser/TokenStream.h

@@ -177,6 +177,15 @@ public:
         return 0;
     }
 
+    size_t current_index() const { return m_index; }
+
+    ReadonlySpan<T> tokens_since(size_t start) const
+    {
+        if (start > m_index)
+            return {};
+        return m_tokens.slice(start, m_index - start);
+    }
+
     void dump_all_tokens()
     {
         dbgln("Dumping all tokens:");