Fix frozen string literal warnings on Ruby 3.4

[mattbrictson]

On the recently-released Ruby 3.4, the following warning is printed when using the plist gem:

plist-3.7.1/lib/plist/parser.rb:60: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)

Fix by using "".dup to ensure the string literal is not frozen by default in future Ruby versions.