refactor: migrate decimal validator to ts

[tshuli]

To be merged in after #749

Problem

Part of #7

Solution

Improvements

• Migrates Decimal validation to Typescript

Tests

• Create two decimal fields. Hide one of them behind logic.
• Check that validation on both visible and hidden decimal fields work when submitting programatically
• Decimal input is rejected if it is not within the validation range
• Decimal input is rejected if it has leading zeroes

[karrui]

This PR seems to have both the dateValidator and decimalValidator changes, can help to separate them first before I review?

[liangyuanruo]

don't declare interfaces in a running function

[tshuli]

shifted out

[liangyuanruo]

is the fallback to empty object truly necessary?

[mantariksh]

shouldn't be since customMin and customMax both have defaults, but we should also double check (eg by deleting ValidationOptions on a decimal field and checking if mongoose puts it back automatically)

[tshuli]

removed the fallback

[tshuli]

@liangyuanruo @mantariksh actually can we keep the fallback since there is really no downside; although mongoose restores the default when I tested it manually, the failing tests make me nervous

[liangyuanruo]

would this end up achieving the same logic?

const isFloatOptions = {
  min: customMin,
  max: customMax
}

[tshuli]

IsFloatOptions activates its options based on the presence of each property, hence it is necessary to manually add 'min' and 'max'; added a comment within

[liangyuanruo]

maybe the isFloatOptions variable can be inlined into isFloat directly?

[tshuli]

As above, no because the properties must be conditionally added