feat(asdf): add 3rd-party fallback completion loader for "asdf"

The interface "asdf completion bash" has been documented for the first
time in asdf-0.16.0.

https://github.com/asdf-vm/asdf/blob/2114f1ebc15b3f1653228c8b416f759898a58dc7/docs/guide/getting-started.md#set-up-shell-completions-optional
asdf-vm/asdf#1826

Author: akinomyoga

completions/.gitignore

@@ -26,6 +26,7 @@
 /_argo
 /arm-koji
 /asciidoc.py
+/_asdf
 /_atlas
 /_atmos
 /autoheader

completions/Makefile.am

@@ -582,6 +582,7 @@ CLEANFILES = \
 	_argo \
 	arm-koji \
 	asciidoc.py \
+	_asdf \
 	_atlas \
 	_atmos \
 	autoheader \
@@ -1180,6 +1181,7 @@ symlinks: $(DATA)
 		_aqua \
 		_arduino-cli \
 		_argo \
+		_asdf \
 		_atlas \
 		_atmos \
 		_bashbot \