Allow jsonext to be defined in adapt.json

[oliverfoster]

Subject of the issue

jsonext must currently be specified as grunt dev --jsonext=txt https://github.com/adaptlearning/adapt_framework/blob/9dce12c48e5ccf0c8faa52e5daf61efa2afe405e/GRUNT.md?plain=1#L17

It would be good to be able to add it to adapt.json as:

{
  "jsonext": "txt"
}

As with some of the other properties:

https://github.com/adaptlearning/adapt_framework/blob/9dce12c48e5ccf0c8faa52e5daf61efa2afe405e/grunt/helpers.js#L147-L172

https://github.com/adaptlearning/adapt_framework/blob/9dce12c48e5ccf0c8faa52e5daf61efa2afe405e/grunt/helpers.js#L207-L214