devel/transient: Update 0.11.0 => 0.12.0, take maintainership, improve port

Changelog:
https://github.com/magit/transient/blob/v0.12.0/CHANGELOG

Improve port:

• Add option INFO.
• DOCS, INFO and PDF build-depend on GNU sed.
• Fix build with DOCS off.
• BUILD_DEPENDS==RUN_DEPENDS => move dependencies to _BR_DEPENDS and use: BUILD_DEPENDS=_BR_DEPENDS RUN_DEPENDS=_BR_DEPENDS
• Move makeinfo from USES to DOCS_USES.
• Split long line MAKE_ENV and remove "+".
• Move pkg-plist to PLIST_FILES in Makefile - 3 files in list only.
• Remove CONFLICTS_INSTALL=transient-devel-emacs* - devel/transient-devel remove from tree 2025-09-01.

PR: 292945
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>