Add a cross-platform "open" command to lib-core

[ezzatron]

Linux uses xdg-open, macOS uses open. Targets that open URLs or files should work cross-platform.

[jmalloc]

FYI, Iggy tried to solve this on his own machine by aliasing open to xdg-open, but that doesn't work because as per https://stackoverflow.com/a/60022277 ...

Aliases are only supported in interactive shells by default.

Also, any code run in a subprocess (i.e. any command you run which is not a shell builtin) will be unable to change anything in the parent process (aliases, variables, etc). So you can't change the behavior of your current shell with make, or a script, or any other external command (except if you cause the shell to cooperate, for example by using a shell builtin which evaluates code printed by a subprocess - eval and source are two common ways to make the shell cooperate).

Not that an alias would form part of our solution, but I figured I'd post it here in case we experiment the same way and are equally baffled.

Edit:

[ignatius] also, xdg-open needs to be issued as xdg-open &>/dev/null