devel/transient: Update to 0.13.4, improve port

Remove custom build targets in favor of OPT_ALL_TARGET/ALL_TARGET.
Add distribution for CSS and Javascript assets, modify install
targets to install them.
Patch documentation to use installed assets.
Patch GNU sed syntax away, and drop dependency.
Adjust version constraint on compat.el and cond-let.

PR: 295645
Approved by: submitter is maintainer
Changes: https://raw.githubusercontent.com/magit/transient/refs/tags/v0.13.4/CHANGELOG