Add OpenExternal, a function for opening files in the OS

[ChrisJefferson]

Add a function "OpenExternal", which opens files in other apps in the file system. While I like the names used by existing packages (like Splash), I also don't want to clash with those existing packages.

Rather than base this on the existing code used for help, I wrote a new function. This does remove the customizability, but also means we can open any file type. Also, in my experience, nowadays the "default" operating system method of opening files is what I want.

This also does not implement the functionality provided by (for example) semigroups, where files such as '.dot' and '.tex' files are compiled before being opened.

I'm very happy for suggestions on naming / functionality / etc. I thought having a PR would give a place to discuss such issues.

Discussed in #4272

[wilfwilson]

I like this feature, and I think OpenExternal is a decent name for it.

[wilfwilson]

I'd be happy to merge this, I just wanted to sanity check before doing so:

• Have you tested the code yourself on the various operating systems? I've tried it on my Mac and I can confirm that it worked as expected.
• Is it possible to add tests in GAP for OpenExternal? I presume such a test might be too brittle (and what would success even look like?) but if possible to do robustly, it could at least catch the case that open/explorer.exe/xdg-open... become renamed or whatever.

[fingolfin]

I don't see a good way to test this automatically in our CI. So I'd just merge this as-is, but I'll wait to give a chance for more comments.

[fingolfin]

OK, I don't want to whine, but I have to repeat one of my pet mantras, namely that "Exec is a really bad function, we should never have added it like that". Besides not even allowing to check the return value, it also forces us to do silly quoting tricks like the above; and this is not even complete, a filename with a " in it will break it. We really should think about adding an alternative to Exec which is a bit better (doesn't go through a shell and hence has no quoting issues, and gives access to return value, and optionally allows for suppressing or redirecting output.

[fingolfin]

(But of course none of this is required for this PR!)