Speed up derived applications like Panel by speeding up Path Parameter

Panel=`main`, param=2.0.1, python=3.10,os=linux

In https://github.com/holoviz/panel/issues/5852 I'm reporting that for a simple app Panel is spending 2.75% of its load time resolving file paths.

Upon investigation I can see its because the paths to css files etc is hold by a `Path` parameter. And that implementation resolves a string path to a pathlib Path every time its asked for its value.


**Please add some kind of caching to only resolve once**