Implement the util type IsFunction<T> which accepts type T and return true if it's function or false if it is not.

// expected true for all statements below:
IsFunction<() => void> 
IsFunction<(n: number) => boolean> 
IsFunction<(n: number, a: string[]) => Promise<void>> 

// expected false for all statements below:
IsFunction<12> 
IsFunction<'abc'>