Description:
Currently, Tailcall allows developers to define custom scalars. However, there's no built-in way to add validations on those custom scalars. I propose creating a @validate directive which takes js function as input and can be applied on scalars. The js function inside @validate should return either true or false. If it returns false, the user should receive a Evaluation error when an invalid value is passed into the scalar.

Expected Behavior:

• The @validate directive should be applicable to custom scalar types.
• The directive js function should return true for valid values and false for invalid values.
• If the directive returns false, a Evaluation error should be thrown to indicate the issue to the user.

Tasks:

• Implement a @validate directive that developers can use on custom scalars.
• @Validate accepts a js function as input.
• errors should follow the formatting guidelines of Tailcall Evaluation Errors

Example

scalar EvenNumber @validate(js: "isEven")

function isEven(num) {
	return num % 2 === 0
}