Normative: Fully specify legal escape sequences in RegExp capture gro…

…up names (#1869)

This commit makes the Early Errors for RegExpIdentifierStart and RegExpIdentifierPart fully specified, with the semantics that Unicode escape sequences of the form `\u LeadSurrogate \u TrailSurrogate` as well as \u { CodePoint }` are legal in named capture group names for both Unicode and non-Unicode regular expressions.

This commit thus makes legal all of the following:
 - `/(?<\ud835\udc9c>.)/`
 - `/(?<\ud835\udc9c>.)/u`
 - `/(?<\u{1d49c}>.)/`
 - `/(?<\u{1d49c}>.)/u`
 - `/(?<𝒜>)/`
 - `/(?<𝒜>)/u`

Fixes #1861

spec.html

@@ -30885,7 +30885,7 @@ <h2>Syntax</h2>
           `.`
           `\` AtomEscape[?U, ?N]
           CharacterClass[?U]
-          `(` GroupSpecifier[?U] Disjunction[?U, ?N] `)`
+          `(` GroupSpecifier Disjunction[?U, ?N] `)`
           `(` `?` `:` Disjunction[?U, ?N] `)`
 
         SyntaxCharacter :: one of
@@ -30898,7 +30898,7 @@ <h2>Syntax</h2>
           DecimalEscape
           CharacterClassEscape[?U]
           CharacterEscape[?U]
-          [+N] `k` GroupName[?U]
+          [+N] `k` GroupName
 
         CharacterEscape[U] ::
           ControlEscape
@@ -30915,27 +30915,27 @@ <h2>Syntax</h2>
           `a` `b` `c` `d` `e` `f` `g` `h` `i` `j` `k` `l` `m` `n` `o` `p` `q` `r` `s` `t` `u` `v` `w` `x` `y` `z`
           `A` `B` `C` `D` `E` `F` `G` `H` `I` `J` `K` `L` `M` `N` `O` `P` `Q` `R` `S` `T` `U` `V` `W` `X` `Y` `Z`
 
-        GroupSpecifier[U] ::
+        GroupSpecifier ::
           [empty]
-          `?` GroupName[?U]
+          `?` GroupName
 
-        GroupName[U] ::
-          `&lt;` RegExpIdentifierName[?U] `&gt;`
+        GroupName ::
+          `&lt;` RegExpIdentifierName `&gt;`
 
-        RegExpIdentifierName[U] ::
-          RegExpIdentifierStart[?U]
-          RegExpIdentifierName[?U] RegExpIdentifierPart[?U]
+        RegExpIdentifierName ::
+          RegExpIdentifierStart
+          RegExpIdentifierName RegExpIdentifierPart
 
-        RegExpIdentifierStart[U] ::
+        RegExpIdentifierStart ::
           UnicodeIDStart
           `$`
           `_`
-          `\` RegExpUnicodeEscapeSequence[?U]
+          `\` RegExpUnicodeEscapeSequence[+U]
 
-        RegExpIdentifierPart[U] ::
+        RegExpIdentifierPart ::
           UnicodeIDContinue
           `$`
-          `\` RegExpUnicodeEscapeSequence[?U]
+          `\` RegExpUnicodeEscapeSequence[+U]
           &lt;ZWNJ&gt;
           &lt;ZWJ&gt;
 
@@ -31086,13 +31086,13 @@ <h1>Static Semantics: Early Errors</h1>
         <emu-grammar>RegExpIdentifierStart[U] :: `\` RegExpUnicodeEscapeSequence[?U]</emu-grammar>
         <ul>
           <li>
-            It is a Syntax Error if the SV of |RegExpUnicodeEscapeSequence| is none of *"$"*, or *"_"*, or the UTF16Encoding of a code point matched by the |UnicodeIDStart| lexical grammar production.
+            It is a Syntax Error if the CharacterValue of |RegExpUnicodeEscapeSequence| is none of *"$"*, or *"_"*, or the UTF16Encoding of a code point matched by the |UnicodeIDStart| lexical grammar production.
           </li>
         </ul>
         <emu-grammar>RegExpIdentifierPart[U] :: `\` RegExpUnicodeEscapeSequence[?U]</emu-grammar>
         <ul>
           <li>
-            It is a Syntax Error if the SV of |RegExpUnicodeEscapeSequence| is none of *"$"*, or *"_"*, or the UTF16Encoding of either &lt;ZWNJ&gt; or &lt;ZWJ&gt;, or the UTF16Encoding of a Unicode code point that would be matched by the |UnicodeIDContinue| lexical grammar production.
+            It is a Syntax Error if the CharacterValue of |RegExpUnicodeEscapeSequence| is none of *"$"*, or *"_"*, or the UTF16Encoding of either &lt;ZWNJ&gt; or &lt;ZWJ&gt;, or the UTF16Encoding of a Unicode code point that would be matched by the |UnicodeIDContinue| lexical grammar production.
           </li>
         </ul>
         <emu-grammar>UnicodePropertyValueExpression :: UnicodePropertyName `=` UnicodePropertyValue</emu-grammar>