Use acf/is_field_key to validate our non-field_ keys

[JiveDig]

After a discussion Liam from WPE/ACF, i found out that we wrongly register fields with keys that do not start with field_, which could potentially cause performance and other issues.

Elliot mentioned this requirement back in 2013 (https://support.advancedcustomfields.com/forums/topic/creating-theme-fields-generating-a-key/#post-6663), but somehow we missed it.

It wasn't really an issue until ACF 6.3.x when block validation stuff was added.