[scripts] Add vcpkg_completions.zsh (#18274)

* Add vcpkg_completions.zsh

* Update scripts/vcpkg_completion.zsh

scripts/vcpkg_completion.zsh

@@ -0,0 +1,9 @@
+
+_vcpkg_completions()
+{
+  local vcpkg_executable=${COMP_WORDS[0]}
+  local remaining_command_line=${COMP_LINE:(${#vcpkg_executable}+1)}
+  COMPREPLY=($(${vcpkg_executable} autocomplete "${remaining_command_line}" -- 2>/dev/null))
+}
+
+complete -F _vcpkg_completions vcpkg