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>

config/miaur.conf.example

@@ -17,3 +17,5 @@ Noconfirm = false
 # SearchBy = name-desc
 # Editor = nvim
 # Bat = bat
+# RepoUrl = https://github.com/WHC2006/miaur.git
+# Branch = main