add ext example for custom smart syntax

src/ext/_examples/CustomSmartSyntax/.htaccess

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+deny from all

src/ext/_examples/CustomSmartSyntax/extension.json

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+{
+    "bundleId": "CustomSmartSyntax",
+    "name": "Custom Smart Syntax",
+    "version": "1.0",
+    "description": "Prototype to write you own smart syntax parser"
+}

src/ext/_examples/CustomSmartSyntax/loader.php

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+<?php declare(strict_types=1);
+
+/*
+    This file is a part of myTinyTodo.
+    (C) Copyright 2023 Max Pozdeev <maxpozdeev@gmail.com>
+    Licensed under the GNU GPL version 2 or any later. See file COPYRIGHT for details.
+*/
+
+if (!defined('MTTPATH')) {
+    die("Unexpected usage.");
+}
+
+function mtt_ext_customsmartsyntax_instance(): MTTExtension
+{
+    return new CustomSmartSyntaxExtension();
+}
+
+class CustomSmartSyntaxExtension extends MTTExtension implements MTTFilterInterface
+{
+    //the same as dir name
+    const bundleId = 'CustomSmartSyntax';
+
+    function init() {
+        \MTTFilterCenter::addFilterForAction('parseSmartSyntax', $this);
+    }
+
+    // parseSmartSyntax
+    function filter($title, &$out)
+    {
+        $a = [
+            'prio' => 0,         // int: -1 .. 2
+            'title' => $title,   // string
+            'tags' => '',        // string: "tag1, tag2, tag3,..."
+            'duedate' => null,   // string: "Y-m-d" format
+        ];
+
+        // This filter just overwrites results of default parser
+
+        $out = $a;
+    }
+}