editors/slime: Update to version 2.23

Other port changes:

• use the upstream build target
• pacify portlint
• conform to the Porter's Handbook "Order of Variables" recommendations

Upstream changes: https://github.com/slime/slime/blob/master/NEWS

PR: 234977
Approved by: joe@thrallingpenguin.com (maintainer)
Differential Revision: https://reviews.freebsd.org/D18845