lunar-client-bin: add package (3.7.12-1)

Upstream moved to an "ow" electron-updater channel with a filename
suffix; latest_version checks both channels (vercmp picks the winner) and
refresh_checksums keeps the _appimage filename in sync. Also clean up
downloaded .AppImage files like the other source leftovers.

.gitignore

@@ -5,3 +5,4 @@
 */src/
 */pkg/
 *.deb
+*.AppImage