Add getTextFieldValues to buildFieldTrait to handle multi-valued text-based fields

Problem/Motivation

We have `BuildFieldTrait::getTextFieldValue`, but then it's used on multivalued fields, it returns comma separated values, which is an issue if the text itself contains comma.

Proposed resolution

Add `BuildFieldTrait::getTextFieldValues` to return multiple values in an array. The pairing of `BuildFieldTrait::getTextFieldValue` & `BuildFieldTrait::getTextFieldValues` will be analogous to `BuildFieldTrait::getReferencedEntityFromField` and `BuildFieldTrait::getReferencedEntitiesFromField`

API changes

- No changes to existing methods.
- A new method `BuildFieldTrait::getTextFieldValues()` will now returns an array of values for multi-field text-based fields.