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

https://docs.astral.sh/uv/getting-started/installation/#shell-autocompletion

Author: scop

Diff for: completions/Makefile.am

@@ -455,6 +455,7 @@ bashcomp_DATA = 2to3 \
 		useradd \
 		userdel \
 		usermod \
+		_uvx \
 		valgrind \
 		_vault \
 		vipw \

Diff for: completions/_uvx

@@ -0,0 +1,8 @@
+# 3rd party completion loader for commands emitting        -*- shell-script -*-
+# their completion using "$cmd --generate-shell-completion bash".
+#
+# This serves as a fallback in case the completion is not installed otherwise.
+
+eval -- "$("$1" --generate-shell-completion bash 2>/dev/null)"
+
+# ex: filetype=sh

Diff for: test/fallback/completions/Makefile.am

@@ -47,6 +47,7 @@ EXTRA_DIST = \
 	udevadm \
 	umount \
 	umount.linux \
+	uvx \
 	vault \
 	write \
 	xm \

Diff for: test/fallback/completions/uvx

@@ -0,0 +1 @@
+../../../completions/_uvx