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.

_helpers.bash

@@ -4,22 +4,22 @@ function basedir() {
 
 function _source_file_if_exists() {
 	if [ -r "$1" ]; then
-		test -n "$DEBUG_BASHRC" && echo "-- Sourcing file $1"
+		test -n "${DEBUG_BASHRC}" && echo "-- Sourcing file $1"
 		source "$1"
 	fi
 }
 
 function _source_dir_files() {
 	if [ -d "$1" ]; then
-		test -n "$DEBUG_BASHRC" && echo "-- Sourcing files in directory $1"
+		test -n "${DEBUG_BASHRC}" && echo "-- Sourcing files in directory $1"
 
 		# Safe loops for empty dirs
 		shopt -s nullglob
 
 		for file in "$1"/*; do
-			if [ -e "$file" ]; then
-				test -n "$DEBUG_BASHRC" && echo "    * sourcing file $file"
-				source "$file"
+			if [ -e "${file}" ]; then
+				test -n "${DEBUG_BASHRC}" && echo "    * sourcing file ${file}"
+				source "${file}"
 			fi
 		done
 
@@ -30,13 +30,13 @@ function _source_dir_files() {
 
 # Source : https://superuser.com/questions/39751/add-directory-to-path-if-its-not-already-there
 _path_add() {
-	if [ -d "$1" ] && [[ ":$PATH:" != *":$1:"* ]]; then
-		PATH="${PATH:+"$PATH:"}$1"
+	if [ -d "$1" ] && [[ ":${PATH}:" != *":$1:"* ]]; then
+		PATH="${PATH:+"${PATH}:"}$1"
 	fi
 }
 
 _prompt_command_add() {
-	if [ -n "$1" ] && [[ ":$PROMPT_COMMAND:" != *":$1:"* ]]; then
-		PROMPT_COMMAND="${PROMPT_COMMAND:+"$PROMPT_COMMAND;"}$1"
+	if [ -n "$1" ] && [[ ":${PROMPT_COMMAND}:" != *":$1:"* ]]; then
+		PROMPT_COMMAND="${PROMPT_COMMAND:+"${PROMPT_COMMAND};"}$1"
 	fi
 }