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

https://github.com/defenseunicorns/zarf/blob/7cf34a5cf370a8eb23d2a22e63410d4e51eb5614/docs/2-the-zarf-cli/100-cli-commands/zarf_completion_bash.md

Author: scop

completions/.gitignore

@@ -349,4 +349,5 @@
 /xpovray
 /xvnc4viewer
 /ypcat
+/_zarf
 /_zitadel

completions/Makefile.am

@@ -913,6 +913,7 @@ CLEANFILES = \
 	xpovray \
 	xvnc4viewer \
 	ypcat \
+	_zarf \
 	_zitadel
 
 symlinks: $(DATA)
@@ -1050,6 +1051,7 @@ symlinks: $(DATA)
 		_upctl \
 		_vacuum \
 		_virtctl \
+		_zarf \
 		_zitadel
 	$(ss) gpgv \
 		gpgv2