feat: builder api

[mixvar]

closes #93

published as 0.2.8-beta.2 (see sandbox)

example:

import { FormSchemaBuilder, FormFields, FormValidatorBuiler } from "@virtuslab/formts";

const Schema = new FormSchemaBuilder()
  .fields({ answer: FormFields.string() })
  .errors<string>()
  .build();

const validator = new FormValidatorBuilder(Schema)
  .validate(
    Schema.answer,
    val => (val === "" ? "Required!" : null),
    val => (val !== "42" ? "Wrong answer!" : null)
  )
  .build();