`Node#>` operator does not work in a `DocumentFragment`

[flavorjones]

Originally reported on the mailing list by @MaxLustig and at #1853 which was self-closed.

Failing example:

#! /usr/bin/env ruby

require 'nokogiri'

html = <<-EOHTML
    <div class="section header" id=1>
      <div class="subsection header">sub 1</div>
      <div class="subsection header">sub 2</div>
    </div>
    <div class="section header" id=2>
      <div class="subsection header">sub 3</div>
      <div class="subsection header">sub 4</div>
    </div>
EOHTML

doc = Nokogiri::HTML.fragment html

parent = doc.at_css("body")

x = parent > ".header"
# /home/flavorjones/foo.rb:22:in `<main>': undefined method `>' for nil:NilClass (NoMethodError)

We should move the definition of Node#> into Searchable and make sure it works for DocumentFragment. I'll further note that that implementation might be fraught because of the well-documented xpath-on-root-nodes issues (see #572), but this is essentially a failing test that we need to make work at some point.