[ruby/irb] Refactor RubyLex#process_literal_type

(https://github.com/ruby/irb/pull/350)

Simplify part of regex ``[_a-zA-Z0-9]`` with equivalent shorthand ``\w``.
Replace case-when with match ``$1`` or default value ``?"``, making intention more clear.

lib/irb/ruby-lex.rb

@@ -782,13 +782,8 @@ class RubyLex
     when :on_qsymbols_beg then ?]
     when :on_symbols_beg  then ?]
     when :on_heredoc_beg
-      start_token&.tok =~ /<<[-~]?(['"`])[_a-zA-Z0-9]+\1/
-      case $1
-      when ?" then ?"
-      when ?' then ?'
-      when ?` then ?`
-      else         ?"
-      end
+      start_token&.tok =~ /<<[-~]?(['"`])\w+\1/
+      $1 || ?"
     else
       nil
     end