add an enum for 'view'/'form' display types

Problem/Motivation

There are methods like setDisplayConfigurable() and setDisplayOptions() which take a parameter which is only one of 'form' or 'view'. Anything else fails silently.

Making this take an enum would improve DX. (I for one always think its 'display' and not 'view.)

Steps to reproduce

Proposed resolution

Add an enum for this value.
Change all the methods that take this type of value to have a parameter type of string|ThisEnum
Add BC handling to complain about a string being passed

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet