feat: spellcheck input (#6446)

Reviewed-by: Mm2PL <mm2pl+gh@kotmisia.pl>
Reviewed-by: pajlada <rasmus.karlsson@pajlada.com>
Reviewed-by: fourtf <tf.four@gmail.com>

mocks/include/mocks/EmptyApplication.hpp

@@ -208,6 +208,11 @@ public:
         return nullptr;
     }
 
+    SpellChecker *getSpellChecker() override
+    {
+        return nullptr;
+    }
+
 #ifdef CHATTERINO_HAVE_PLUGINS
     PluginController *getPlugins() override
     {