Fix treesit-node-field-name and friends (bug#66674)

So turns out ts_node_field_name_for_child takes a named node index,
but we were passing it normal index that counts both named and
anonymous nodes.  That's what makes the field name all wrong in
treesit explorer.

* doc/lispref/parsing.texi:
(Accessing Node Information): Update docstring.
* lisp/treesit.el (treesit-node-index): Add some unrelated comment.
(treesit-node-field-name): Get named node index rather than all node
index.
* src/treesit.c (Ftreesit_node_field_name_for_child): Update
docstring, use ts_node_named_child_count.

doc/lispref/parsing.texi

@@ -1015,8 +1015,8 @@ This function returns the field name of the @var{n}'th child of
 @var{node}.  It returns @code{nil} if there is no @var{n}'th child, or
 the @var{n}'th child doesn't have a field name.
 
-Note that @var{n} counts both named and anonymous children, and
-@var{n} can be negative, e.g., @minus{}1 represents the last child.
+Note that @var{n} counts named nodes only, and @var{n} can be
+negative, e.g., @minus{}1 represents the last child.
 @end defun
 
 @defun treesit-node-child-count node &optional named

lisp/treesit.el

@@ -360,6 +360,7 @@ If NAMED is non-nil, collect named child only."
   "Return the index of NODE in its parent.
 If NAMED is non-nil, count named child only."
   (let ((count 0))
+    ;; TODO: Use next-sibling as it's more efficient.
     (while (setq node (treesit-node-prev-sibling node named))
       (cl-incf count))
     count))
@@ -367,7 +368,7 @@ If NAMED is non-nil, count named child only."
 (defun treesit-node-field-name (node)
   "Return the field name of NODE as a child of its parent."
   (when-let ((parent (treesit-node-parent node))
-             (idx (treesit-node-index node)))
+             (idx (treesit-node-index node t)))
     (treesit-node-field-name-for-child parent idx)))
 
 ;;; Query API supplement

src/treesit.c

@@ -2015,9 +2015,8 @@ DEFUN ("treesit-node-field-name-for-child",
 Return nil if there's no Nth child, or if it has no field.
 If NODE is nil, return nil.
 
-N counts all children, i.e., named ones and anonymous ones.
-
-N could be negative, e.g., -1 represents the last child.  */)
+Note that N counts named nodes only.  Also, N could be negative, e.g.,
+-1 represents the last child.  */)
   (Lisp_Object node, Lisp_Object n)
 {
   if (NILP (node))
@@ -2031,7 +2030,7 @@ N could be negative, e.g., -1 represents the last child.  */)
 
   /* Process negative index.  */
   if (idx < 0)
-    idx = ts_node_child_count (treesit_node) + idx;
+    idx = ts_node_named_child_count (treesit_node) + idx;
   if (idx < 0)
     return Qnil;
   if (idx > UINT32_MAX)