zstd: Move to a new zstd package

Zstd is a discrete, self-contained system component. To match how we
package zlib, bzip2 and xz, move it to its own package, with a separate
lib package.

Add the new package to the minimal set, since this is a core component
that users expect to be installed.

This change adds a new package to the system so, until we have a proper
policy on how to handle this in release/stable branches, it should not
be MFC'd.

MFC after: never
Reviewed by: bapt
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53603