Add support for Plasma (KDE) 6

[ondrakucera]

Several notes worth mentioning:

• This should be good enough until Plasma 7 is released years from now (Plasma 6 was released roughly ten years after Plasma 5).
• However, it would be nicer if, in such a case, the script would fail (instead of doing nothing as it does now). I didn't want to go there but perhaps adding a "default" branch calling exit_failure_operation_failed would be the way to go?
• I'm expecting kde-open to be present under Plasma 6. This is the case on Arch Linux; I can't speak for other distributions. However, looking at how it is packaged on Arch, I don't see any weird customization, so I expect that if you build kde-cli-tools elsewhere, you will have kde-open, too.

Fixes #336.

[ondrakucera]

Now I see the script is actually taken from https://gitlab.freedesktop.org/xdg/xdg-utils/-/blob/master/scripts/xdg-open.in, which, in its current version, contains the same lines for Plasma 6 as my patch does. But it may also differ elsewhere, co it may make sense to just completely copy its newest version.

[sindresorhus]

Yes, would be great if you could update it to the latest version.

[ondrakucera]

Done (#338).