newline: support PROMPT_ON_NEWLINE

The newline segment will now show ├─ when using
`POWERLEVEL9K_PROMPT_ON_NEWLINE`

Note: This can be overridden via `POWERLEVEL9K_MULTILINE_FIRST_PROMPT_PREFIX`

powerlevel9k.zsh-theme

@@ -339,14 +339,18 @@ prompt_background_jobs() {
 
 # A newline in your prompt, so you can segments on multiple lines.
 prompt_newline() {
-  local lws
+  local lws newline
   [[ "$1" == "right" ]] && return
+  newline=$'\n'
   lws=$POWERLEVEL9K_WHITESPACE_BETWEEN_LEFT_SEGMENTS
+  if [[ "$POWERLEVEL9K_PROMPT_ON_NEWLINE" == true ]]; then
+    newline="${newline}$(print_icon 'MULTILINE_NEWLINE_PROMPT_PREFIX')"
+  fi
   POWERLEVEL9K_WHITESPACE_BETWEEN_LEFT_SEGMENTS=
   "$1_prompt_segment" \
     "$0" \
     "$2" \
-    "NONE" "NONE" $'\n'
+    "NONE" "NONE" "${newline}"
   POWERLEVEL9K_WHITESPACE_BETWEEN_LEFT_SEGMENTS=$lws
 }