Narrow the API of _text_sanitize() and make it public

As part of #1950632: Create a FieldDefinitionInterface and use it for formatters and widgets, we want to decouple formatters from $field and $instance config entities to allow greater reuse. Which means, we also need to decouple the helper functions that they call from these variables. _text_sanitize() is one such function. There's no need to pass it an entire config entity when all it needs is a single setting.

Also, considering all the places in core that invoke it, I don't think we can pretend it's a private function, so renaming it.