drop php7.3 support, make use of 7.4 features

ext/numeric_score/info.php

@@ -4,11 +4,11 @@ class NumericScoreInfo extends ExtensionInfo
 {
     public const KEY = "numeric_score";
 
-    public $key = self::KEY;
-    public $name = "Post Scores (Numeric)";
-    public $url = self::SHIMMIE_URL;
-    public $authors = self::SHISH_AUTHOR;
-    public $license = self::LICENSE_GPLV2;
-    public $description = "Allow users to score images";
-    public $documentation ="Each registered user may vote a post +1 or -1, the image's score is the sum of all votes.";
+    public string $key = self::KEY;
+    public string $name = "Post Scores (Numeric)";
+    public string $url = self::SHIMMIE_URL;
+    public array $authors = self::SHISH_AUTHOR;
+    public string $license = self::LICENSE_GPLV2;
+    public string $description = "Allow users to score images";
+    public ?string $documentation ="Each registered user may vote a post +1 or -1, the image's score is the sum of all votes.";
 }

ext/numeric_score/main.php

@@ -2,9 +2,9 @@
 
 class NumericScoreSetEvent extends Event
 {
-    public $image_id;
-    public $user;
-    public $score;
+    public int $image_id;
+    public User $user;
+    public int $score;
 
     public function __construct(int $image_id, User $user, int $score)
     {
@@ -18,7 +18,7 @@ class NumericScoreSetEvent extends Event
 class NumericScore extends Extension
 {
     /** @var NumericScoreTheme */
-    protected $theme;
+    protected ?Themelet $theme;
 
     public function onDisplayingImage(DisplayingImageEvent $event)
     {

ext/numeric_score/theme.php

@@ -94,7 +94,7 @@ class NumericScoreTheme extends Themelet
     }
 
 
-    public function get_help_html()
+    public function get_help_html(): string
     {
         return '<p>Search for posts that have received numeric scores by the score or by the scorer.</p>
         <div class="command_example">