strip end of line CR from show_source

Description

The ruby parser considers CR to be a whitespace character, but IRB does not seem to do that.
This may result in an extra ^M at the end of a line when viewing the source from IRB.

How to reproduce

Save the following source code in CRLF newline on Windows and execute it on WSL.

def desc
  "this file has CRLF newline"
end
binding.irb

$ ruby /mnt/r/crlf.rb

From: /mnt/r/crlf.rb @ line 4 :

    1: def desc^M
    2:   "this file has CRLF newline"^M
    3: end^M
 => 4: binding.irb^M

irb(main):001> show_source "desc"

From: /mnt/r/crlf.rb:1

def desc^M
  "this file has CRLF newline"^M
end^M

=> nil
irb(main):002>

Start at the show_source implementation and reproduce the issue with a Ruby source file using CRLF newlines on Windows or WSL. Trace how source lines are read and displayed, then verify that show_source no longer prints a trailing ^M while preserving normal source output.