feat(incus): add 3rd party fallback completion loader

https://github.com/lxc/incus/blob/ae41ae87afc906c815753dab176dd2bb14543269/cmd/incus/completion.go

Author: scop

completions/.gitignore

@@ -131,6 +131,7 @@
 /ifstatus
 /_ignite
 /import
+/_incus
 /_infracost
 /inotifywatch
 /insmod.static

completions/Makefile.am

@@ -661,6 +661,7 @@ CLEANFILES = \
 	ifstatus \
 	_ignite \
 	import \
+	_incus \
 	_infracost \
 	inotifywatch \
 	insmod.static \
@@ -1008,6 +1009,7 @@ symlinks: $(DATA)
 		_goreleaser \
 		_helm \
 		_hugo \
+		_incus \
 		_ignite \
 		_istioctl \
 		_k3s \