Editorial: Link to early error from the relevant grammar productions

It's easy to miss the early error section when reading the grammar. This
adds links to the early error section from the grammar productions which
have relevant early errors.

It's done by breaking up the `<emu-grammar>` into multiple grammars
because it is apparently not possible to put a cross-reference into
the grammarkdown prose.

Closes: #3128

Author: ptomato

spec/abstractops.html

@@ -1108,7 +1108,9 @@ <h1>RFC 9557 / ISO 8601 grammar</h1>
       DateYear :::
         DecimalDigit DecimalDigit DecimalDigit DecimalDigit
         ASCIISign DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit
-
+    </emu-grammar>
+    <p>Note the prohibition on negative zero in <emu-xref href="#sec-temporal-iso8601grammar-static-semantics-early-errors"></emu-xref>.</p>
+    <emu-grammar type="definition">
       DateMonth :::
         `0` NonZeroDigit
         `10`
@@ -1129,10 +1131,14 @@ <h1>RFC 9557 / ISO 8601 grammar</h1>
       DateSpecMonthDay :::
         `--`? DateMonth DateSeparator[+Extended] DateDay
         `--`? DateMonth DateSeparator[~Extended] DateDay
-
+    </emu-grammar>
+    <p>Note the prohibition on invalid combinations of month and day in <emu-xref href="#sec-temporal-iso8601grammar-static-semantics-early-errors"></emu-xref>.</p>
+    <emu-grammar type="definition">
       DateSpec[Extended] :::
         DateYear DateSeparator[?Extended] DateMonth DateSeparator[?Extended] DateDay
-
+    </emu-grammar>
+    <p>Note the prohibition on invalid combinations of month and day in <emu-xref href="#sec-temporal-iso8601grammar-static-semantics-early-errors"></emu-xref>.</p>
+    <emu-grammar type="definition">
       Date :::
         DateSpec[+Extended]
         DateSpec[~Extended]
@@ -1224,7 +1230,9 @@ <h1>RFC 9557 / ISO 8601 grammar</h1>
       AnnotatedTime :::
         TimeDesignator Time DateTimeUTCOffset[~Z]? TimeZoneAnnotation? Annotations?
         Time DateTimeUTCOffset[~Z]? TimeZoneAnnotation? Annotations?
-
+    </emu-grammar>
+    <p>Note the disambiguation between the second alternative without |TimeDesignator|, and |DateSpecMonthDay|/|DateSpecYearMonth| in <emu-xref href="#sec-temporal-iso8601grammar-static-semantics-early-errors"></emu-xref>.</p>
+    <emu-grammar type="definition">
       AnnotatedDateTime[Zoned, TimeRequired] :::
         [~Zoned] DateTime[~Z, ?TimeRequired] TimeZoneAnnotation? Annotations?
         [+Zoned] DateTime[+Z, ?TimeRequired] TimeZoneAnnotation Annotations?