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

https://gitlab.com/gitlab-org/cli/-/tree/main/docs/source/completion

Author: scop

completions/.gitignore

@@ -148,6 +148,7 @@
 /_gitleaks
 /_gitsign
 /gkrellm2
+/_glab
 /_glen
 /_glow
 /gmake

completions/Makefile.am

@@ -716,6 +716,7 @@ CLEANFILES = \
 	_gitleaks \
 	_gitsign \
 	gkrellm2 \
+	_glab \
 	_glen \
 	_glow \
 	gmake \
@@ -1159,7 +1160,7 @@ symlinks: $(DATA)
 	$(ss) geoiplookup \
 		geoiplookup6
 	$(ss) _gh \
-		_infracost
+		_glab _infracost
 	$(ss) gkrellm \
 		gkrellm2
 	$(ss) _golangci-lint \