preview releases as `pwsh-preview` instead of `pwsh`

Expose preview releases as `pwsh-preview`, not `pwsh`.

However, can we support simultaneous installation of a preview release and a stable release? If not, this isn't worth the effort.

Don't forget about `pwsh-np-preview`

## Implementation notes:

* Publish pwsh-preview to a different npm package name, to support parallel installations.
* To keep internal implementation simple, can keep the pwsh symlink, but swap out how it's exposed via package.json bin field.
* Our build script creates a collection of tarballs, then publishes them all via `npm publish` If the tarballs have modified names, they will publish to a different package name automatically. The code that modifies the version can also modify the name.

Should be a really simple tweak to our build.ps1