Allow source to be optional (do not error when file missing)

[MakiseKurisu]

I use nwg-displays to configure temporary display layout. However, its configuration files are only created when running the program once. I use NixOS to manage several machines, and some of them is regularly wiped to experiment, so the first time booting up I will always meet this error message.

This feature also allows some temporary config to simply be deleted to remove it, instead of commenting out, when main config is in a read-only Nix store so you cannot change the source line.

[phonetic112]

# hyprlang noerror true
source = ~/your/file
# hyprlang noerror false