Refactor: update text block conversion to use text block type

[jonwaldstein]

Resolves #

Description

This updates the text block conversion to use the first class TextBlockType.

This is the first in series approach to moving away from the untyped $block->getAttribute() and toward specific block types with typed block attributes in the form of object properties.

This also caught and fixes an issue with the BlockType class so it will always cast values as intended. So if you are expecting a boolean and for some reason the block does not have the attribute it will still return a boolean type instead of null. This makes sure the block type property types are always valid types and can be confidently used in typed methods in the fields api.

Affects

• Custom text block fields in the VFB

Visuals

N/A

Testing Instructions

• Make sure new and existing custom text blocks work as expected on v3 forms

Pre-review Checklist

• Acceptance criteria satisfied and marked in related issue
• Relevant @unreleased tags included in DocBlocks
• Includes unit tests
• Reviewed by the designer (if follows a design)
• Self Review of code and UX completed