Use v RegExp flag instead of u for <input pattern>

This makes the pattern attribute more powerful, enabling the use of RegExp set notation syntax and properties of strings in its values.

Differences with the previous u flag-based behavior:

- [FEATURE] Previously invalid patterns now become valid, e.g.:

    pattern="[\p{ASCII_Hex_Digit}--[Ff]]"
    pattern="\p{RGI_Emoji}"
    pattern="[_\q{a|bc|def}]"

- [BREAKING CHANGE] Some previously valid patterns are now errors, specifically those with a character class including either an unescaped special character ( ) [ ] { } / - \ | or a double punctuator.

- [STATUS QUO] Other previously valid patterns still behave the same. (Other than the above-mentioned features, the v flags only differs in behavior from the u flag w.r.t. case-insensitive matching, but the pattern attribute uses case-sensitive matching.)

Tests: web-platform-tests/wpt#38547.

Fixes #7908.

source

@@ -3032,6 +3032,9 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
     <ul class="brief">
      <li>The <dfn data-x-href="https://tc39.es/proposal-resizablearraybuffer/#sec-isarraybufferviewoutofbounds">IsArrayBufferViewOutOfBounds</dfn> abstract operation</li>
     </ul>
+
+    <p>User agents that support JavaScript must also implement the <cite>RegExp <code data-x="">v</code> flag</cite>
+    proposal. <ref spec=JSREGEXPVFLAG></p>
    </dd>
 
    <dt>WebAssembly</dt>
@@ -50855,7 +50858,8 @@ ldh-str       = &lt; as defined in <a href="https://www.rfc-editor.org/rfc/rfc10
   data-x="concept-fe-values">values</span>, are to be checked.</p>
 
   <p>If specified, the attribute's value must match the JavaScript <i
-  data-x="js-prod-Pattern">Pattern</i><sub><code data-x="">[+UnicodeMode, +N]</code></sub> production.</p>
+  data-x="js-prod-Pattern">Pattern</i><sub><code data-x="">[+UnicodeSetsMode, +N]</code></sub> production. <ref
+  spec=JSREGEXPVFLAG></p>
 
   <div w-nodev>
 
@@ -50871,7 +50875,7 @@ ldh-str       = &lt; as defined in <a href="https://www.rfc-editor.org/rfc/rfc10
    data-x="attr-input-pattern">pattern</code> attribute of the element.</p></li>
 
    <li><p>Let <var>regexpCompletion</var> be <span>RegExpCreate</span>(<var>pattern</var>,
-   "<code data-x="">u</code>"). <ref spec=JAVASCRIPT></p></li>
+   "<code data-x="">v</code>"). <ref spec=JSREGEXPVFLAG></p></li>
 
    <li>
     <p>If <var>regexpCompletion</var> is an <span data-x="Completion Record">abrupt
@@ -50886,7 +50890,7 @@ ldh-str       = &lt; as defined in <a href="https://www.rfc-editor.org/rfc/rfc10
    <var>pattern</var>, followed by "<code data-x="">)$</code>".</p></li>
 
    <li><p>Return ! <span>RegExpCreate</span>(<var>anchoredPattern</var>, "<code
-   data-x="">u</code>").</p></li>
+   data-x="">v</code>").</p></li>
   </ol>
 
   <p class="note">The reasoning behind these steps, instead of just using the value of the <code
@@ -133426,6 +133430,9 @@ INSERT INTERFACES HERE
    <dt id="refsJSJSONMODULES">[JSJSONMODULES]</dt>
    <dd><cite><a href="https://tc39.es/proposal-json-modules/">JSON Modules</a></cite>. Ecma International.</dd>
 
+   <dt id="refsJSREGEXPVFLAG">[JSREGEXPVFLAG]</dt>
+   <dd><cite><a href="https://github.com/tc39/proposal-regexp-v-flag">RegExp <code data-x="">v</code> flag with set notation + properties of strings</a></cite>. Ecma International.</dd>
+
    <dt id="refsJSRESIZABLEBUFFERS">[JSRESIZABLEBUFFERS]</dt>
    <dd><cite><a href="https://tc39.es/proposal-resizablearraybuffer/">Resizable ArrayBuffer and growable SharedArrayBuffer</a></cite>. Ecma International.</dd>