feat: multi selection. Only integrates with send_selected_to_qflist (#551)

This will not yet work with select actions. More work is needed in that case.

Co-authored-by: Simon Hauser <Simon-Hauser@outlook.de>

lua/telescope/pickers/multi.lua

@@ -0,0 +1,51 @@
+
+local MultiSelect = {}
+MultiSelect.__index = MultiSelect
+
+function MultiSelect:new()
+  return setmetatable({
+    _entries = {}
+  }, MultiSelect)
+end
+
+function MultiSelect:get()
+  local marked_entries = {}
+  for entry, count in pairs(self._entries) do
+    table.insert(marked_entries, {count, entry})
+  end
+
+  table.sort(marked_entries, function(left, right)
+    return left[1] < right[1]
+  end)
+
+  local selections = {}
+  for _, entry in ipairs(marked_entries) do
+    table.insert(selections, entry[2])
+  end
+
+  return selections
+end
+
+function MultiSelect:is_selected(entry)
+  return self._entries[entry]
+end
+
+local multi_select_count = 0
+function MultiSelect:add(entry)
+  multi_select_count = multi_select_count + 1
+  self._entries[entry] = multi_select_count
+end
+
+function MultiSelect:drop(entry)
+  self._entries[entry] = nil
+end
+
+function MultiSelect:toggle(entry)
+  if self:is_selected(entry) then
+    self:drop(entry)
+  else
+    self:add(entry)
+  end
+end
+
+return MultiSelect