Allow lexer to accept tokens that ended at previous positions

* Track lookahead in each tree
* Add 'mark_end' API that external scanners can use

src/runtime/tree.h

@@ -34,6 +34,7 @@ typedef struct Tree {
 
   Length padding;
   Length size;
+  uint32_t bytes_scanned;
 
   TSSymbol symbol;
   TSStateId parse_state;