[feat] engine: implementation of yandex (web, images)

It's set to inactive in settings.yml because of CAPTCHA.  You need to remove
that from the settings.yml to get in use.

Closes: https://github.com/searxng/searxng/issues/961

searx/settings.yml

@@ -1814,6 +1814,22 @@ engines:
     engine: unsplash
     shortcut: us
 
+  - name: yandex
+    engine: yandex
+    categories: general
+    search_type: web
+    shortcut: yd
+    disabled: true
+    inactive: true
+
+  - name: yandex images
+    engine: yandex
+    categories: images
+    search_type: images
+    shortcut: ydi
+    disabled: true
+    inactive: true
+
   - name: yandex music
     engine: yandex_music
     shortcut: ydm