Use TreeCursor to pretty-print :tree-sitter-subtree

[the-mikedavis]

The current :tree-sitter-subtree has a bug for field-names when the field name belongs to an unnamed child node. Take this ruby example:

def self.method_name
  true
end

The subtree given by tree-sitter-cli is:

(singleton_method [2, 0] - [4, 3]
  object: (self [2, 4] - [2, 8])
  name: (identifier [2, 9] - [2, 20])
  body: (body_statement [3, 2] - [3, 6]
    (true [3, 2] - [3, 6])))

But the :tree-sitter-subtree output was

(singleton_method
  object: (self)
  body: (identifier)
  (body_statement (true)))

The singleton_method rule defines the name and body fields in an unnamed helper rule _method_rest and the old implementation of pretty_print_tree_impl would pass the field_name down from the named singleton_method node.

To fix it we switch to the TreeCursor API which is recommended by the tree-sitter docs for traversing the tree. TreeCursor::field_name accurately determines the field name for the current cursor position even when the node is unnamed.