Add ldcup (ldc2 version manager) step

[kassane]

What does this PR do

This adds support for ldcup (ldc2 version manager). Although not officially provided by the ldc-developers group, it is community based to facilitate ldc2 usage.
src: https://github.com/kassane/ldcup

The tool doesn't support other D compilers (dmd/gdc).

Standards checklist

• The PR title is descriptive
• I have read CONTRIBUTING.md
• Optional: I have tested the code myself
• If this PR introduces new user-facing messages they are translated

For new steps

• Optional: Topgrade skips this step where needed
• Optional: The --dry-run option works with this step
• Optional: The --yes option works with this step if it is supported by
  the underlying command

dry-run output

 ./target/release/topgrade --dry-run      

── 12:41:48 - System update ────────────────────────────────────────────────────
Dry running: /usr/bin/trizen -Syu

── 12:41:48 - Flatpak User Packages ────────────────────────────────────────────
Dry running: /usr/bin/flatpak update --user

── 12:41:48 - Flatpak System Packages ──────────────────────────────────────────
Dry running: /usr/bin/flatpak update --system

── 12:41:48 - Nix ──────────────────────────────────────────────────────────────
Dry running: /usr/bin/nix-channel --update
Dry running: /usr/bin/nix --version

── 12:41:48 - oh-my-zsh ────────────────────────────────────────────────────────
Dry running: zsh /home/kassane/.oh-my-zsh/tools/upgrade.sh

── 12:41:48 - TLDR ─────────────────────────────────────────────────────────────
Dry running: /usr/bin/tldr --update

── 12:41:48 - rustup ───────────────────────────────────────────────────────────
Dry running: /usr/bin/rustup update

── 12:41:48 - Cargo ────────────────────────────────────────────────────────────
cargo-update isn't installed so Topgrade can't upgrade cargo packages.
Install cargo-update by running `cargo install cargo-update`

── 12:41:48 - Visual Studio Code extensions ────────────────────────────────────
Dry running: /usr/bin/code --update-extensions

── 12:41:48 - Helix ────────────────────────────────────────────────────────────
Dry running: /usr/bin/helix --grammar fetch
Dry running: /usr/bin/helix --grammar build

── 12:41:48 - Containers ───────────────────────────────────────────────────────
Dry running: /usr/bin/docker pull moby/buildkit:buildx-stable-1 --platform linux/amd64

── 12:41:49 - Lensfun's database update ────────────────────────────────────────
Dry running: /usr/bin/lensfun-update-data 

── 12:41:49 - zigup ────────────────────────────────────────────────────────────
Dry running: /home/kassane/.local/bin/zigup fetch master

── 12:41:49 - ldcup ────────────────────────────────────────────────────────────
Dry running: /home/kassane/.dlang/ldcup install master

── 12:41:49 - Summary ──────────────────────────────────────────────────────────
System update: OK
config-update: OK
Flatpak: OK
oh-my-zsh: OK
TLDR: OK
rustup: OK
Visual Studio Code extensions: OK
helix: OK
Containers: OK
Lensfun's database update: OK
zigup: OK
ldcup: OK

Pacman backup configuration files found:
/etc/pacman.d/mirrorlist.pacnew
/etc/makepkg.conf.d/rust.conf.pacnew
/etc/bash.bashrc.pacnew
/etc/mkinitcpio.conf.pacnew
/etc/sudoers.pacnew
/etc/locale.gen.pacnew
/etc/pacman.conf.pacnew

If you developed a feature or a bug fix for someone else and you do not have the
means to test it, please tag this person here.