[mod] add deepl translation engine

This implements the Deepl Translation engine. It works nearly like lingva but
directly to the deepl API.  This api only needs a to-lang, from-lang is a fake
by now.

There is a free option to use [1].

[1] https://www.deepl.com/pro-api?cta=header-pro-api for registering a free account.

searx/settings.yml

@@ -1626,6 +1626,15 @@ engines:
     engine: seznam
     disabled: true
 
+  # - name: deepl
+  #   engine: deepl
+  #   shortcut: dpl
+  #   # You can use the engine using the official stable API, but you need an API key
+  #   # See: https://www.deepl.com/pro-api?cta=header-pro-api
+  #   api_key: ''  # required!
+  #   timeout: 5.0
+  #   disabled: true
+
   - name: mojeek
     shortcut: mjk
     engine: xpath