Expose standalone JSON path builder @ ExpressionBuilder.

[igalklebanov]

Hey 👋

As mentioned in #440, we can expose support for building JSON paths without a reference arrow (just the '$.key' string). Most of the implementation is already in place.

Why is this important? Well, since we won't support all JSON functions out there, and some of them expect JSON paths as arguments, currently consumers can only provide a non type-safe literal.

Possible API:

jsonPath<RE extends ReferenceExpression<DB, TB>>(ref: RE): JSONPathBuilder<ExtractTypeFromReferenceExpression<DB, TB, RE>>