[+] Adds schema editor for data definitions - see ticket #116:

[PhilipSkinner]

https://github.com/PhilipSkinner/elemental-lowcode/issues/116

The schema editor is based on the json-schema-editor vue component,
but modified to correct a few bugs and to change the language
used. A few features were also disabled. See the original source
here:

https://github.com/giscafer/vue-json-schema-editor-visual