Rename stack_right_position -> stack_total_tree_size

I want to re-use the stack data structure for storing the
re-usable nodes from the previous parse tree during an edit.
In this case, the stack won't conceptually start at position
zero, so the name 'right_position' doesn't make sense.

src/runtime/stack.c

@@ -47,7 +47,7 @@ void ts_stack_shrink(TSStack *stack, size_t new_size) {
   stack->size = new_size;
 }
 
-TSLength ts_stack_right_position(const TSStack *stack) {
+TSLength ts_stack_total_tree_size(const TSStack *stack) {
   TSLength result = ts_length_zero();
   for (size_t i = 0; i < stack->size; i++) {
     TSTree *node = stack->entries[i].node;