Support the new 4.0 definition of isArray

[Orta Therox (orta)]

This PR relates to microsoft/TypeScript#39258

Effectively, it brings your isArray type check inline with the version in this PR ^

Example:

// Original version, uses any exportfunctionisArrayOld(array: any): array is any[]{returnArray.isArray(array);}// New, funky-looking, but retains readonly safelyfunctionisArray<T>(array: T|{}): array is Textendsreadonlyany[] ? (unknownextendsT ? never : readonlyany[]) : any[]{returnArray.isArray(array);}constmutableArray: string[]=[]constimmutableArray: readonlystring[]=[]if(isArray(mutableArray)){mutableArray.push("")}// Fails correctlyif(isArray(immutableArray)){immutableArray.push("")// would error here}/// Previous behaviorif(isArrayOld(mutableArray)){mutableArray.push("")}if(isArrayOld(immutableArray)){immutableArray.push("")// No error on pushing to an readonly array}

Playground Link

[Matt Bierner (mjbvz)]

Thanks!