[scripts/posh-vcpkg] Change TabExpansion to Register-ArgumentCompleter #1788

Job	Run time
format	3m 1s
json-schema	1m 0s
builds / build (windows-2022, windows-ci)	19m 6s
builds / build (ubuntu-20.04, linux-ci)	10m 40s
builds / build (macos-13, macos-ci)	14m 31s
	48m 18s

Provide feedback