Support importing Taskwarrior v2.x data files (#3724)

This should ease the pain of upgrading from v2.x to v3.x.

src/commands/CmdImportV2.h

@@ -0,0 +1,46 @@
+////////////////////////////////////////////////////////////////////////////////
+//
+// Copyright 2006 - 2021, Tomas Babej, Paul Beckingham, Federico Hernandez.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+// THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+//
+// https://www.opensource.org/licenses/mit-license.php
+//
+////////////////////////////////////////////////////////////////////////////////
+
+#ifndef INCLUDED_CMDIMPORTV2
+#define INCLUDED_CMDIMPORTV2
+
+#include <Command.h>
+#include <JSON.h>
+
+#include <string>
+#include <unordered_map>
+
+class CmdImportV2 : public Command {
+ public:
+  CmdImportV2();
+  int execute(std::string &);
+
+ private:
+  int import(const std::vector<std::map<std::string, std::string>> &task_data);
+};
+
+#endif
+////////////////////////////////////////////////////////////////////////////////