Provide namespace for promisyfing

[dalechyn]

child_process.d.ts has next namespace for util.promisify:

// NOTE: This namespace provides design-time support for util.promisify. Exported members do not exist at runtime.
namespace exec {
        function __promisify__(command: string): PromiseWithChild<{ stdout: string, stderr: string }>;
        function __promisify__(command: string, options: { encoding: "buffer" | null } & ExecOptions): PromiseWithChild<{ stdout: Buffer, stderr: Buffer }>;
        function __promisify__(command: string, options: { encoding: BufferEncoding } & ExecOptions): PromiseWithChild<{ stdout: string, stderr: string }>;
        function __promisify__(command: string, options: ExecOptions): PromiseWithChild<{ stdout: string, stderr: string }>;
        function __promisify__(command: string, options?: ({ encoding?: string | null } & ExecOptions) | null): PromiseWithChild<{ stdout: string | Buffer, stderr: string | Buffer }>;
    }

It allows wrapping error-first contracts in promises.
I've tested it on my application with having module augmentation.

declare module 'sudo-prompt' {
  // NOTE: This namespace provides design-time support for util.promisify. Exported members do not exist at runtime.
  namespace exec {
    function __promisify__(command: string): PromiseWithChild<{ stdout: string; stderr: string }>
    function __promisify__(
      command: string,
      options: { encoding: 'buffer' | null } & ExecOptions
    ): PromiseWithChild<{ stdout: Buffer; stderr: Buffer }>
    function __promisify__(
      command: string,
      options: { encoding: BufferEncoding } & ExecOptions
    ): PromiseWithChild<{ stdout: string; stderr: string }>
    function __promisify__(
      command: string,
      options: ExecOptions
    ): PromiseWithChild<{ stdout: string; stderr: string }>
    function __promisify__(
      command: string,
      options?: ({ encoding?: string | null } & ExecOptions) | null
    ): PromiseWithChild<{ stdout: string | Buffer; stderr: string | Buffer }>
  }
}

And it works!