Level-based functions

The level-based version of BetweenFlags is needed for things like finding functions, where then "end" of a function cannot be confused with the "end" of an if statement inside the function. Therefore, the "level" corresponding to that function should be zero both on the opening and closing of the function.

Examples:

Consider trying to grab all functions defined in a file.

  s_i = ""
  s_i = string(s_i, "\n", "Some text")
  s_i = string(s_i, "\n", "if something")
  s_i = string(s_i, "\n", "  function myfunc()")
  s_i = string(s_i, "\n", "    more stuff")
  s_i = string(s_i, "\n", "    if something")
  s_i = string(s_i, "\n", "      print('something')")
  s_i = string(s_i, "\n", "    else")
  s_i = string(s_i, "\n", "      print('not something')")
  s_i = string(s_i, "\n", "    end")
  s_i = string(s_i, "\n", "    for something")
  s_i = string(s_i, "\n", "      print('something')")
  s_i = string(s_i, "\n", "    else")
  s_i = string(s_i, "\n", "      print('not something')")
  s_i = string(s_i, "\n", "    end")
  s_i = string(s_i, "\n", "    more stuff")
  s_i = string(s_i, "\n", "  end")
  s_i = string(s_i, "\n", "end")
  s_i = string(s_i, "\n", "more text")

  word_boundaries_left = ["\n", " ", ";"]
  word_boundaries_right = ["\n", " ", ";"]
  word_boundaries_right_if = [" ", ";"]

  FS_outer = FlagSet(
    Flag("function", word_boundaries_left, word_boundaries_right),
    Flag("end",      word_boundaries_left, word_boundaries_right)
  )

  FS_inner = [
  FlagSet(
    Flag("if",       word_boundaries_left, word_boundaries_right_if),
    Flag("end",      word_boundaries_left, word_boundaries_right)
  ),
  FlagSet(
    Flag("for",      word_boundaries_left, word_boundaries_right),
    Flag("end",      word_boundaries_left, word_boundaries_right)
  )]

  L_o = get_level(s_i, FS_outer, FS_inner)
  print("\n -------------- results from complex example: \n")
  print(L_o[1])
  print("\n --------------\n")

 -------------- results from complex example:
 function myfunc()
    more stuff
    if something
      print('something')
    else
      print('not something')
    end
    for something
      print('something')
    else
      print('not something')
    end
    more stuff
  end

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