Add cl-selfupdate (self-update for CL executables via GitHub/GitLab Releases)

[atgreen]

cl-selfupdate lets a delivered Common Lisp executable check for, download, and
apply newer versions of itself from GitHub or GitLab Releases. It handles
release discovery, semantic-version comparison, archive extraction, and
validation, with pluggable HTTP backends (Dexador or Drakma).

Sources: https://github.com/atgreen/cl-selfupdate
Docs/README: https://github.com/atgreen/cl-selfupdate#readme
License: MIT

Suggested source.txt:
latest-github-release https://github.com/atgreen/cl-selfupdate.git

Dependencies (jsown, cl-semver, chipz, archive, zip, quri, ironclad, cl-ppcre,
alexandria, flexi-streams) are already in Quicklisp.