Add a fish-like UX to zsh

This adds and configures the following packages:

* `zsh-history-substring-search` provides a substring search through history and suggests possible arguments
* `zsh-autosuggestions` gives suggestions during typing
* `zsh-syntax-highlighting` colors commands based on their syntax

Resolves #521

mkosi.conf.d/10-packages-cli.conf

@@ -59,4 +59,7 @@ Packages=
     ydotool # Basic TUI and GUI app automation
     zip # the `zip` CLI archiving tool
     zsh
+    zsh-autosuggestions # fish-like suggestions
     zsh-completions
+    zsh-history-substring-search # fish-like history search
+    zsh-syntax-highlighting # fish-like syntax highlighting

mkosi.extra/usr/share/zsh/plugins/kde-linux-default/kde-linux-default-zshrc.zsh

@@ -79,6 +79,9 @@ compinit
 # Disable tab cycling through completions.
 zstyle ':completion:*' menu no
 
+# Make completions case-insensitive.
+zstyle ':completion:*' matcher-list 'm:{a-z}={A-Z}'
+
 # Add colored output for various commands.
 alias ls='ls --color=auto'
 alias grep='grep --color=auto'
@@ -100,3 +103,12 @@ add-zsh-hook preexec auto_chmod
 # Wrap journal logs viewed in terminal rather than truncating; friendlier for reading
 # and copying
 export SYSTEMD_LESS=FRXMK
+
+# Add autosuggestions, substring history search and syntax highlighting.
+source /usr/share/zsh/plugins/zsh-autosuggestions/zsh-autosuggestions.zsh
+source /usr/share/zsh/plugins/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh
+source /usr/share/zsh/plugins/zsh-history-substring-search/zsh-history-substring-search.zsh
+
+# Bind Ctrl+↑ and Ctrl+↓ keybinds to substring history search.
+bindkey '^[[1;5A' history-substring-search-up # Ctrl+↑
+bindkey '^[[1;5B' history-substring-search-down # Ctrl+↓