Virtual IO for testing

[sergey-shandar]

type Io = {
  readFileSync: ...;
  writeFileSync: ...;
  readFileAsync: ...;
  writeFileAsync: ...;
  fetch: ...;
  document: ...; 
  console_log: ...;
}

/** typedef {{
*  readonly readFileSync: ...;
*  readonly writeFileSync: ...;
*  readonly readFileAsync: ...;
*  readonly writeFileAsync: ...;
*  readonly fetch: ...;
*  readonly document: ...; 
*  readonly console_log: ...;
* }} Io
*/