Reduce memory usages on Praat lexer

[tancnle]

This helps to reduce memory consumption by allowing objects to be garbage collected. This can be verified by running rake check:memory task.

retained objects by file
        <snipped>
         15  rouge/lib/rouge/lexers/mosel.rb                                          
         14  ruby/lib/lib/ruby/2.7.0/set.rb                                           
         13  rouge/lib/rouge/lexers/common_lisp.rb                                    
-        13  rouge/lib/rouge/lexers/praat.rb                                          
         13  rouge/lib/rouge/themes/igor_pro.rb                                                                                                                             
         12  rouge/lib/rouge/lexers/haxe.rb                                           
         10  rouge/lib/rouge/guessers/util.rb    
          <snipped>