Beginless ranges previously raised TypeError for this case,
except for string ranges, which had unexpected behavior:

  ('a'..'z').include?('ww') # false
  (..'z').include?('ww') # previously true, now TypeError

Use of include? with endless ranges could previously result
in an infinite loop.

This splits off a range_string_cover_internal function from
range_include_internal.

Fixes [Bug #18580]