Instanceof issue fatal

[Stouffi]

closes #1001

Here is a codesandbox to illustrates the problem encountered: https://codesandbox.io/s/youthful-sunset-6jn810?file=/src/index.test.ts

An issue with instanceof schema should be fatal in order to prevent running refinement on a value which might be incompatible with the provided instance type.

This PR introduces an additional optional parameter in the custom signature to specify wether the issue should be fatal.

declare const custom: <T>(
  check?: (data: unknown) => any,
  params?: Parameters<ZodTypeAny["refine"]>[1],
  fatal?: boolean
) => ZodType<T>

However it would be better to specify it as an object to specify more options in the future.

interface CustomOptions {
  fatal: boolean
}

declare const custom: <T>(
  check?: (data: unknown) => any,
  params?: Parameters<ZodTypeAny["refine"]>[1],
  opts?: Partial<CustomOptions>
) => ZodType<T>

Let me know if this PR needs improvements to meet the guidelines.

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[colinhacks]

Immaculate PR, thanks!