Returns a new string copied from +self+, with trailing characters possibly removed:

When +line_sep+ is <tt>"\n"</tt>, removes the last one or two characters
if they are <tt>"\r"</tt>, <tt>"\n"</tt>, or <tt>"\r\n"</tt>
(but not <tt>"\n\r"</tt>):

  $/                    # => "\n"
  "abc\r".chomp         # => "abc"
  "abc\n".chomp         # => "abc"
  "abc\r\n".chomp       # => "abc"
  "abc\n\r".chomp       # => "abc\n"
  "こんにちは\r\n".chomp  # => "こんにちは"

When +line_sep+ is <tt>''</tt> (an empty string),
removes multiple trailing occurrences of <tt>"\n"</tt> or <tt>"\r\n"</tt>
(but not <tt>"\r"</tt> or <tt>"\n\r"</tt>):

  "abc\n\n\n".chomp('')           # => "abc"
  "abc\r\n\r\n\r\n".chomp('')     # => "abc"
  "abc\n\n\r\n\r\n\n\n".chomp('') # => "abc"
  "abc\n\r\n\r\n\r".chomp('')     # => "abc\n\r\n\r\n\r"
  "abc\r\r\r".chomp('')           # => "abc\r\r\r"

When +line_sep+ is neither <tt>"\n"</tt> nor <tt>''</tt>,
removes a single trailing line separator if there is one:

  'abcd'.chomp('cd')   # => "ab"
  'abcdcd'.chomp('cd') # => "abcd"
  'abcd'.chomp('xx')   # => "abcd"

Related: see {Converting to New String}[rdoc-ref:String@Converting+to+New+String].
