Register pde:// browser protocol for .pdez and .pdex files on Linux

[benfry]

Starting with Processing 4.2, you can use pde:// URLs to open .pdex and .pdez files from the internet and the local machine (see #559). These are implemented on macOS and Windows, but I ran out of time to implement and test them properly on Linux.

To implement, the necessary incantations should be added to the Linux install script. This looks like a reasonably up to date resource on how to make it happen. Or this older document was used as the basis for the Windows and macOS versions, and includes a Linux implementation as well.

Please help!

[peonix0]

Could you please assign this issue to me. I have already finished half of the work. I'll raise a PR soon.