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.

aliases.d/gentoo.bash

@@ -1,4 +1,4 @@
-if [ -f /etc/gentoo-release -a "$UID" -ne 0 ]
+if [ -f /etc/gentoo-release -a "${UID}" -ne 0 ]
 then
 	alias emerge='sudo emerge'
 	alias etc-update='sudo etc-update'