VS: Add policy to build custom commands concurrently

In commit 33c15ae2 (VS: Build custom commands concurrently when
possible, 2023-01-19, v3.26.0-rc1~56^2) we added `BuildInParallel` to
custom commands in `.vcxproj` files, but that had to be reverted by
commit abb1c121 (VS: Revert "Build custom commands concurrently when
possible", 2023-03-07, v3.26.0-rc6~3^2) because some projects may have
custom commands that accidentally rely on serial execution in MSBuild.

Add a policy to use `BuildInParallel` for custom commands in projects
that have been updated to set the policy to `NEW`.

Fixes: #18405