Use TreeCursor to pretty-print :tree-sitter-subtree (#4606)

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.

[TreeCursor]: https://docs.rs/tree-sitter/0.20.9/tree_sitter/struct.TreeCursor.html

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