Use pwsh shebang, and Add-Type over LoadFile

[Hurricos]

This PR mixes two things that perhaps ought not to be:

• use a pwsh shebang, for Unix compat (using e.g. https://github.com/PowerShell/powershell/releases)
• for loading .dll's, use Add-Type over LoadFile.

Again, might serve better as two commits, but eh.