Add json helper

[thekid]

This helper can be used to transport values from PHP to JavaScript:

<script>
  let person = {{&json person}};
  console.log(person.name);
</script>

• Takes care of quoting and escaping strings
• Escapes forward slashes to prevent parsing problems with closing script tags inside strings
• Has an option format=true to pretty-print the JSON

See also https://stackoverflow.com/questions/10232574/need-handlebars-js-to-render-object-data-instead-of-object-object and https://laravel.com/docs/master/blade#rendering-json

[thekid]

This does not add a new dependency, xp-forge/json was already required by xp-forge/frontend.

[thekid]

Released in https://github.com/xp-forge/handlebars-templates/releases/tag/v4.3.0