Add support for .sources files

[kneekoo]

OS: Linux Mint 21.3 Cinnamon
Problem: The mintsource GUI is unaware of APT sources saved in .source files and doesn't show them at all.

The APT sources for the WineHQ repository migrated from a .list file to a .sources file. Here is its contents:

nicu@Nicu-PC:/etc/apt/sources.list.d$ more winehq-jammy.sources
Types: deb
URIs: https://dl.winehq.org/wine-builds/ubuntu
Suites: jammy
Components: main
Architectures: amd64 i386
Signed-By: /etc/apt/keyrings/winehq-archive.key