[enh] add SVG favicon href="favicon.svg" type="image/svg+xml"

Add link::

    <link rel="icon" href="favicon.png" sizes="any">
    <link rel="icon" href="favicon.svg" type="image/svg+xml">

Modern browsers should request::

    INFO  werkzeug   : 127.0.0.1 - - [28/Nov/2021 17:03:07] "GET /static/themes/simple/img/favicon.svg HTTP/1.1" 200 -

[1] https://github.com/audreyfeldroy/favicon-cheat-sheet#svg-file
[2] https://dev.to/masakudamatsu/favicon-nightmare-how-to-maintain-sanity-3al7

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/static/themes/simple/gruntfile.js

@@ -150,7 +150,15 @@ module.exports = function(grunt) {
           '<%= _templates %>/__common__/searxng-wordmark.min.svg': '<%= _brand %>/searxng-wordmark.svg',
           'img/searxng.svg': '<%= _brand %>/searxng.svg'
         }
-      }
+      },
+      favicon: {
+        options: {
+          svgo: ['--config', 'svg4favicon.svgo.js']
+        },
+        files: {
+          'img/favicon.svg': '<%= _brand %>/searxng-wordmark.svg'
+        }
+      },
     },
     svg2png: {
       favicon: {