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refactor(field-validators): introduce scaffolding & reference implementations for section, short text & long text fields (#409)

Email submission validators are crucial for preventing spoof submissions, and need to be migrated to TypeScript as part of push towards type safety. The existing implementation is problematic to port over however, due to

* Type consistency
* Misuse of the Inheritance pattern
* Lack of Codebase clarity

Solution

* Composition over Inheritance - Encourage code reuse in order to compose larger validation functions from smaller ones. For example, Table validation can be composed from Text and Number validator functions.

* Scaffolding for further migration - Perform full submission validation for all form field responses, instead of throwing an exception on a short-circuit basis upon encountering the first error that occurs. The hope is to use functional programming utilities so that all responses can be evaluated in order to obtain as much information as possible before rejecting a submission.

Changes:

* refactor: convert field-validation/index.ts and associated tests to typescript
* refactor(field-validators): textFieldValidatorConstructor sectionValidatorConstructor
* fix: only non-empty answers to be validated
* refactor: delete AnswerNotAllowedValidator, TextValidator, unused PlaceholderValidator
* refactor(ValidationOptions): Extract common validation options enum from short/long text fields, and use it in textValidator

Addresses #7

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