Add theme.json for current themes

This won't be used in 4.7 branch, but the website will soon use this
metadata.

See https://github.com/phpmyadmin/phpmyadmin/pull/13261

Signed-off-by: Michal Čihař <michal@cihar.com>

mhn/info.inc.php

@@ -1,3 +1,3 @@
 <?php
 $theme_name = 'mhn';
-$theme_full_version = '1.2';
+$theme_full_version = '1.3';

mhn/theme.json

@@ -0,0 +1,8 @@
+{
+    "name": "mhn",
+    "version": "1.3",
+    "author": "Mohan Khadka",
+    "url": "http://www.mohankhadka.com.np/",
+    "supports": ["4.7"]
+}
+