Editorial: add more lookahead-restrictions to NotEscapeSequence

Specifically, add a [lookahead <! HexDigit] restriction to the
two right-hand sides:
    u { NotCodePoint
    u { CodePoint
to force them to consume all the HexDigits there.
Otherwise, the NotEscapeSequence could end 'early',
causing 'TemplateCharacters' to be ambiguous.

spec.html

@@ -10702,8 +10702,8 @@ <h2>Syntax</h2>
           `u` HexDigit HexDigit [lookahead &lt;! HexDigit]
           `u` HexDigit HexDigit HexDigit [lookahead &lt;! HexDigit]
           `u` `{` [lookahead &lt;! HexDigit]
-          `u` `{` NotCodePoint
-          `u` `{` CodePoint [lookahead != `}`]
+          `u` `{` NotCodePoint [lookahead &lt;! HexDigit]
+          `u` `{` CodePoint [lookahead &lt;! HexDigit] [lookahead != `}`]
 
         NotCodePoint ::
           HexDigits [&gt; but only if MV of HexDigits &gt; 0x10FFFF ]
@@ -10845,10 +10845,10 @@ <h1>Static Semantics: TV and TRV</h1>
             The TRV of <emu-grammar>NotEscapeSequence :: `u` `{` [lookahead &lt;! HexDigit]</emu-grammar> is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET).
           </li>
           <li>
-            The TRV of <emu-grammar>NotEscapeSequence :: `u` `{` NotCodePoint</emu-grammar> is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET) followed by the code units of the TRV of |NotCodePoint|.
+            The TRV of <emu-grammar>NotEscapeSequence :: `u` `{` NotCodePoint [lookahead &lt;! HexDigit]</emu-grammar> is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET) followed by the code units of the TRV of |NotCodePoint|.
           </li>
           <li>
-            The TRV of <emu-grammar>NotEscapeSequence :: `u` `{` CodePoint [lookahead != `}`]</emu-grammar> is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET) followed by the code units of the TRV of |CodePoint|.
+            The TRV of <emu-grammar>NotEscapeSequence :: `u` `{` CodePoint [lookahead &lt;! HexDigit] [lookahead != `}`]</emu-grammar> is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B (LEFT CURLY BRACKET) followed by the code units of the TRV of |CodePoint|.
           </li>
           <li>
             The TRV of <emu-grammar>DecimalDigit :: one of `0` `1` `2` `3` `4` `5` `6` `7` `8` `9`</emu-grammar> is the SV of the |SourceCharacter| that is that single code point.