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

doc/telescope.txt

@@ -184,6 +184,15 @@ telescope.setup({opts})                                    *telescope.setup()*
 
         Default: '  '
 
+                                            *telescope.defaults.multi_icon*
+    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: '+'
+
                                           *telescope.defaults.initial_mode*
     initial_mode: ~
         Determines in which mode telescope starts. Valid Keys: