title: How to create default value in field using sanity.io?
body: <p>Is it possible to add to sanity field with default value? How can I extend it? I want to create some fields with default variables. For example I have this code:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>export default {
    name: 'name',
    title: 'name',
    type: 'document',
    fields: [
      {
        name: 'title',
        title: 'title',
        type: 'string',
        validation: Rule =&gt; Rule.required(),
      },
      {
        name: 'key',
        title: 'key',
        type: 'slug',
        options: {
          source: 'title',
          maxLength: 96
        }
      },
      {
        name: 'geo',
        title: 'geo',
        type: 'geopoint',
        //default: {"lat": 1,"lng": 2}
      },
      {
        name: 'tel',
        title: 'tel',
        type: 'string',
        //default: '122334554565'
      },
    ],
    preview: {
      select: {
        title: 'title'
      }
    }
  }</code></pre>
  </div>
  </div>
  </p>