style: Normalize variable expansions to ${var} across the repo

Replace bare $var with ${var} everywhere it was still used, matching
the convention already established in rc.d/prompt and rc.d/history.
Positional parameters ($1, $@, $#) and special variables ($?, $$, $-)
are left as-is, consistent with existing usage in _helpers.bash.

rc.d/broot is auto-generated by the broot binary and will likely be
overwritten with its original formatting on the next broot update.

libs/term.bash

@@ -1,5 +1,5 @@
 function term_change_title {
-	case $TERM in
+	case ${TERM} in
 	# Change the window title of X terminals
 	xterm* | rxvt* | urxvt* | Eterm)
 		echo -ne "\033]0;${1}\007"