feat: multiselect icon (#1572)

* feat: add `multi_icon` option to pickers and corresponding highlight

* feat: allow `multi_icon` to be any length

* fix: adjust `selection_caret` highlighting

lua/telescope/config.lua

@@ -245,6 +245,18 @@ append(
   Default: '  ']]
 )
 
+append(
+  "multi_icon",
+  "+",
+  [[
+  Symbol to add in front of a multi-selected result entry.
+  Replaces final character of |telescope.defaults.selection_caret| and
+  |telescope.defaults.entry_prefix| as appropriate.
+  To have no icon, set to the empty string.
+
+  Default: '+']]
+)
+
 append(
   "initial_mode",
   "insert",