[typescript] fix type aliases generation

[mildsunrise]

For schemas of primitive types:

components:
  schemas:
    Timestamp:
      description: UTC timestamp in milliseconds since epoch
      type: number

It was previously generating invalid code:

/**
 * UTC timestamp in milliseconds since epoch
 * @export
 */
export interface Timestamp {
}

Now it generates:

/**
 * UTC timestamp in milliseconds since epoch
 * @export
 */
export type Timestamp = number

[mildsunrise]

(also applies to arrays)

[HugoMario]

LGTM, thanks a lot @mildsunrise !!!