bug#50506: 28.0.50; display-line-numbers equivalent for linum-format?

[Michael Gallagher (CIRES/NOAA)]

linum mode has several performance issues but has the convenient ability
to format the line numbers in a way chosen by the user. It would be
wonderful to be able to add a "separator" character of choosing to
display-line-numbers-mode. I would be happy to try to contribute code
but I'm unsure of where to start with such a task. Thanks everyone for
all the incredible work, it has greatly enabled my research as a scientist


In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.30, cairo 
version 1.17.4)
 of 2021-08-18 built on slytux
Repository revision: 0d5ff75e944e765dce0e3a93ebdefa4fb6d5f356
Repository branch: master
System Description: Arch Linux

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --mandir=/usr/share/man --with-gameuser=:games
 --with-sound=alsa --with-modules --without-gconf --without-gsettings
 --with-libsystemd --localstatedir=/var --with-compress-install
 --with-x-toolkit --without-sound --without-xpm --without-imagemagick
 --with-mailutils --with-pop --without-xml2 --without-json
 --without-harfbuzz --without-libotf --without-m17n-flt
 --without-toolkit-scroll-bars --without-xaw3d --without-xim
 --without-gpm --without-dbus --without-gsettings --without-selinux
 --with-modules --without-makeinfo --without-libgmp --with-cairo
 --disable-largefile --enable-link-time-optimization
 --with-native-compilation --enable-link-time-optimization
 '--program-transform-name=s/\([ec]tags\)/\1.emacs/' 'CFLAGS=-pipe
 -march=native -mtune=native -Ofast -fomit-frame-pointer
 -fno-finite-math-only -flto -fuse-linker-plugin' 'LDFLAGS=-pipe
 -march=native -mtune=native -Ofast -fno-finite-math-only''

Configured features:
ACL CAIRO FREETYPE GIF GLIB GNUTLS JPEG LCMS2 LIBSYSTEMD MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP THREADS TIFF X11
XDBE XIM GTK3 ZLIB

Important settings:
  value of $LANG: C
  locale-coding-system: utf-8

Memory information:
((conses 16 572042 263662)
 (symbols 48 39163 3)
 (strings 32 142081 23304)
 (string-bytes 1 5048285)
 (vectors 16 72877)
 (vector-slots 8 1976472 259237)
 (floats 8 1268 582)
 (intervals 56 18624 15713)
 (buffers 992 25))


-- 
Michael Gallagher, PhD
CIRES Research Scientist
Polar Observations and Processes Team (ESRL/NOAA/PSD)