Re-organize the shells' initialization logic

- unify ~/.config/shell/aliases.sh and ~/.config/shell/utils.sh
  into a common ~/.config/shell/init.sh
- split the contents of ~/.config/shell/aliases.sh
  into the folder ~/.config/shell/aliases.d
- split the contents of ~/.config/shell/utils.sh
  into the folder ~/.config/shell/utils.d

.config/shell/aliases.d/git.sh

@@ -0,0 +1,24 @@
+# All git aliases (with < 7 characters) become shell aliases with a "g" prefix
+
+
+_command_exists() {
+    command -v "$1" 1>/dev/null 2>&1
+}
+
+
+if _command_exists git; then
+    alias g='git'
+
+    for al in $(git internal-aliases); do
+        [ ${#al} -lt 7 ] && eval "alias g$al='git $al'"
+    done
+
+    # Check if a 'main' branch exists in place of a 'master' branch
+    git_main_branch() {
+        if [[ -n "$(git branch --list main)" ]]; then
+           echo 'main'
+        else
+           echo 'master'
+        fi
+    }
+fi