isCompactJson = (value) => {
  return ArrayBuffer.isView(value) && value.subarray(0, 1).toString() === '['
}
Type guard for determining what kind of JSON fixture we have.

This is _very_ loose, but the main difference is that the "Compact JSON"
format is a JSON array and `DirectoryJSON` is an JSON object. Further, the
former is always "compact" (like the output of `JSON.stringify()` without
indents).

@param {DirectoryJSON | Uint8Array} value
@returns {value is JsonUint8Array<SnapshotNode>}
@see {@link https://jsonjoy.com/specs/compact-json}