Migrate field validators and associated tests to Typescript about formsg HOT 5 CLOSED

Assigning to myself first to spearhead a sample FP implementation that should simplify what we have today.

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@jia1's PR on email domain restriction #143 may have some conflicts with the refactor; take note

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Potential idea - do we want to refactor this away from class-based implementation to a functional implementation instead? the rationale being that validators are best implemented without having to hold state. also, we have good test coverage so it ought to be fairly safe to do so.

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Reference implementation has been added in #409 . As usual, please add your name to the validator if you are working on it:

Validator

• Attachment (@tshuli #1181)
• Checkbox (@tshuli #1225)
• Date (@tshuli #749)
• Decimal (@tshuli #750)
• Dropdown (@tshuli #861)
• Email (@tshuli #1126 )
• HomeNo (@tshuli #704)
• Mobile (@tshuli #713)
• Nric (@liangyuanruo #639 )
• Number (@tshuli #1145 )
• Radiobutton (@tshuli #723)
• Rating (@tshuli #724)
• Section (@liangyuanruo #409)
• Table (@tshuli #1316 )
• Text (@liangyuanruo #409)

Tests

For safety of email submissions, these are to be ported from Jasmine to Jest after its corresponding validator has been migrated and running in production for at least a week.

• Attachment
• Checkbox
• Date
• Decimal
• Dropdown
• Email
• HomeNo
• Mobile
• Nric
• Number
• Radiobutton
• Rating
• Section
• Table
• Text

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self-assigning a couple first to get more chances to practice :)

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