[steamhelper] Add new port

[tomh500]

Add new port: steamhelper

This PR adds the steamhelper port, a C++ utility library for Steam-related auxiliary functions.

• Changes comply with the maintainer guide.
• The packaged project shows strong association with the chosen port name.
  • The port name follows the 'GitHubOrg-GitHubRepo' form or equivalent Owner-Project form.
• Optional dependencies of the build are all controlled by the port.
• The versioning scheme in vcpkg.json matches what upstream says.
• The license declaration in vcpkg.json matches what upstream says.
• The installed as the "copyright" file matches what upstream says.
• The source code of the component installed comes from an authoritative source.
• The generated "usage text" is brief and accurate.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Exactly one version is added in each modified versions file.

[tomh500]

@microsoft-github-policy-service agree

[BillyONeal]

It looks like you didn't write a portfile.cmake?

[tomh500]

forgot it, now portfile.cmake has been added