Add self-update command to refresh miaur from Git

Introduce `miaur self-update` / `--self-update` that clones RepoUrl@Branch,
detects user vs system install prefix, and reinstalls in place. Configurable
via conf/env; supports --noconfirm, --repo-url, and --branch.

Co-authored-by: Frank Vitrano <WHC2006@users.noreply.github.com>

lib/config.sh

@@ -22,6 +22,8 @@ MIAUR_MAKEPKG="${MIAUR_MAKEPKG:-makepkg}"
 MIAUR_PACMAN="${MIAUR_PACMAN:-pacman}"
 MIAUR_BAT="${MIAUR_BAT:-bat}"
 MIAUR_EDITOR="${VISUAL:-${EDITOR:-nano}}"
+MIAUR_REPO="${MIAUR_REPO:-https://github.com/WHC2006/miaur.git}"
+MIAUR_BRANCH="${MIAUR_BRANCH:-main}"
 
 _bool_val() {
 	case "${1,,}" in
@@ -73,6 +75,8 @@ miaur_load_config() {
 		Pacman) MIAUR_PACMAN=$val ;;
 		Bat) MIAUR_BAT=$val ;;
 		Editor) MIAUR_EDITOR=$val ;;
+		RepoUrl | MIAUR_REPO) MIAUR_REPO=$val ;;
+		Branch | MIAUR_BRANCH) MIAUR_BRANCH=$val ;;
 		esac
 	done <"$conf"
 }