[yup] InferType should default to nonRequired()

Issue
When creating a schema with Yup, you don't get validation errors on validate if a property is missing; it's non-required by default.

When using InferType<>, it marks all properties as required by default unless explicitly noted as notRequired(), which leads to TS errors in vscode

Sample

import {object, string, InferType} from 'yup'

const schema = object({
  requiredString: string().required(),
  notRequiredString: string()
})

// Some method that takes schema json as an argument
const method = (json: InferType<typeof schema>) => {}

// Error: Property 'notRequiredString' is missing in type '{ requiredString: string; }' but required in type (...)
method({
  requiredString: "done"
})

Expected
The above should show no error since notRequiredString isn't marked as required

• Authors: @dhardtke @vtserman @MoretonBayRC @sseppola @YashdalfTheGray
  @vincentjames501 @robertbullen @sat0yu @deskoh @mauricedb @kalley

Start by locating the Yup TypeScript definitions for InferType and the object schema shown in the issue. Check how an unmarked string field is represented compared with requiredString, then add coverage demonstrating that the inferred notRequiredString property may be omitted while requiredString remains required.