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

[cspotcode]

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