Add a snapcraft.yml for building Snaps

The included snapcraft.yml can be used to build a Snap (the application
format used by Canonical's modern package manager) out of Prism. If the
project wants in the future, Prism can publish these Snaps to the Snap
Store so the Prism Launcher can be installed through the Ubuntu Software
app on vanilla Ubuntu.

I haven't registered the Snap name, so it's currently free for anyone to
reserve. I'd suggest that a Prism developer register the name
prismlauncher at https://snapcraft.io/register-snap to ensure the name
belongs to the project, even if there are no plans of setting up CI to
publish snaps in the short term.

I have also modified JavaUtils.cpp to be able to autodetect the Java
versions included in the Snap, and added "*.snap" to the .gitignore so
the compiled Snap isn't accidentally committed to the repository.

Signed-off-by: Piper McCorkle <contact@piperswe.me>

.gitignore

@@ -47,3 +47,6 @@ result/
 # Flatpak
 .flatpak-builder
 flatbuild
+
+# Snap
+*.snap