[scripts/posh-vcpkg] Change TabExpansion to Register-ArgumentCompleter #1654

Job	Run time
format	3m 35s
json-schema	1m 2s
builds / build (windows-2022, windows-ci)	6m 19s
builds / build (ubuntu-20.04, linux-ci)	4m 55s
builds / build (macos-14, macos-ci)	8m 31s
	24m 22s

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