Normative: Change TimeZone.prototype.toJSON not to call toString

The toJSON method should return the value in the internal slot, instead of
making an observable call to toString.

See: #1808

docs/cookbook/stockExchangeTimeZone.mjs

@@ -130,6 +130,9 @@ class NYSETimeZone extends Temporal.TimeZone {
  toString() {
  return 'NYSE';
  }
+ toJSON() {
+ return 'NYSE';
+ }
 }
 
 const tzNYSE = Object.freeze(new NYSETimeZone());

polyfill/lib/timezone.mjs

@@ -146,7 +146,7 @@ export class TimeZone {
  }
  toJSON() {
  if (!ES.IsTemporalTimeZone(this)) throw new TypeError('invalid receiver');
- return ES.ToString(this);
+ return GetSlot(this, TIMEZONE_ID);
  }
  static from(item) {
  const timeZoneSlotValue = ES.ToTemporalTimeZoneSlotValue(item);

spec/timezone.html

@@ -329,7 +329,7 @@ <h1>Temporal.TimeZone.prototype.toJSON ( )</h1>
  <emu-alg>
  1. Let _timeZone_ be the *this* value.
  1. Perform ? RequireInternalSlot(_timeZone_, [[InitializedTemporalTimeZone]]).
- 1. Return ? ToString(_timeZone_).
+ 1. Return _timeZone_.[[Identifier]].
  </emu-alg>
  </emu-clause>
  </emu-clause>