Clarify distinction btwn tree padding, tree offset, node position

- Node position is public. It represents the node's first character
  index in the document.
- Tree offset is private. It represents the distance between the tree's
  first character index and it's parent's first character index.
- Tree padding is private. It represents the amount of whitespace
  (or other separator characters) immediately preceding the tree.

src/runtime/node.h

@@ -6,7 +6,7 @@
 
 struct TSNode {
   size_t ref_count;
-  size_t start_position;
+  size_t position;
   size_t index;
   const TSTree *content;
   struct TSNode *parent;
@@ -14,7 +14,7 @@ struct TSNode {
 };
 
 TSNode *ts_node_make(const TSTree *tree, TSNode *parent, size_t index,
-                     size_t start_position, const char **names);
+                     size_t position, const char **names);
 TSNode *ts_node_make_root(const TSTree *tree, const char **names);
 
 #endif