Normative: fix SystemTimeZoneIdentifier offsets

This commit adds prose to clarify that if SystemTimeZoneIdentifier
returns an offset time zone identifier, then it should be in normal form
like "-05:00" not "-0500" nor "-05".

No current implementations support returning offset strings in non-
normal form today. So although this commit is normative, it won't
break any existing ECMAScript code nor will it require changes to
any current implementation.

Fixes tc39#2584.

spec/abstractops.html

@@ -1101,6 +1101,9 @@ <h1>ISO 8601 grammar</h1>
       UTCOffsetWithSubMinuteComponents[Extended] :
           Sign Hour[+Padded] TimeSeparator[?Extended] MinuteSecond TimeSeparator[?Extended] MinuteSecond Fraction?
 
+      NormalizedUTCOffset :
+          ASCIISign Hour[+Padded] `:` MinuteSecond
+
       UTCOffsetMinutePrecision :
           Sign Hour[+Padded]
           Sign Hour[+Padded] TimeSeparator[+Extended] MinuteSecond

spec/mainadditions.html

@@ -283,7 +283,7 @@ <h1>SystemTimeZoneIdentifier ( ): a String</h1>
 
       <emu-alg>
         1. If the implementation only supports the UTC time zone, return *"UTC"*.
-        1. Let _systemTimeZoneString_ be the String representing the host environment's current time zone, either: a primary time zone identifier; or an offset time zone identifier.
+        1. [declared="s"] Let _systemTimeZoneString_ be the String representing the host environment's current time zone, either: a primary time zone identifier; or an offset time zone identifier<ins> String _s_ for which ParseText(StringToCodePoints(_s_), |NormalizedUTCOffset|) returns a Parse Node</ins>.
         1. Return _systemTimeZoneString_.
       </emu-alg>