Allow searching for IS (NOT) NULL in default column settings

[Dzieni]

Simple columns like fooBar: {path: 'foo.bar'} could work in a following way:

import {IS_NULL, IS_NOT_NULL} from 'strato-db'

// to search for equality
await model.search({fooBar: 'BAZ'})

// to search for null foo.bar
await model.search({fooBar: IS_NOT_NULL})

// to search for non-null foo.bar
await model.search({fooBar: IS_NULL})

If you implement your own where as a string, then using those symbols would throw.
If you implement your own where as a function, then you are responsible for handling those symbols by yourself.

Thanks to that:

• you have consistent naming in all the projects (with custom columns for null/not null you have a lot of naming conventions to choose => easy to lose consistency)
• code is more DRY

Having such an option should also reduce the need of using where as a function, which is very welcome (where as a function has no easy option to prepare statements)