feat: handle Windows file paths with backslashes in URL parsing

Author: mertcanaltin

url.bs

@@ -544,7 +544,7 @@ U+005F (_).
 </ol>
 
 <p class=note>Of the possible values for the <var>percentEncodeSet</var> argument only two end up
-encoding U+0025 (%) and thus give “roundtripable data”: <a>component percent-encode set</a> and
+encoding U+0025 (%) and thus give "roundtripable data": <a>component percent-encode set</a> and
 <a><code>application/x-www-form-urlencoded</code> percent-encode set</a>. The other values for the
 <var>percentEncodeSet</var> argument — which happen to be used by the <a>URL parser</a> — leave
 U+0025 (%) untouched and as such it needs to be
@@ -2247,8 +2247,19 @@ and then runs these steps:
      <li><p>If <a>c</a> is an <a>ASCII alphanumeric</a>, U+002B (+), U+002D (-), or U+002E (.),
      append <a>c</a>, <a lt="ASCII lowercase">lowercased</a>, to <var>buffer</var>.
 
-     <li>
-      <p>Otherwise, if <a>c</a> is U+003A (:), then:
+<li>
+  <p>Otherwise, if <a>c</a> is U+003A (:) and <var>buffer</var>'s <a for=string>length</a> is 1 and <var>buffer</var>'s first <a for=/>code point</a> is an <a>ASCII alpha</a> and <a>remaining</a> starts with U+005C (\), then:
+
+  <ol>
+   <li><p>Set <var>url</var>'s <a for=url>scheme</a> to "<code>file</code>".
+   <li><p>Set <var>buffer</var> to the empty string.
+   <li><p>Replace the first <a for=/>code point</a> in <a>remaining</a> (U+005C (\)) with U+002F (/).
+   <li><p>If <a>remaining</a> does not start with "<code>//</code>", <a>special-scheme-missing-following-solidus</a> <a>validation error</a>.
+   <li><p>Set <var>state</var> to <a>file state</a>.
+  </ol>
+
+<li>
+  <p>Otherwise, if <a>c</a> is U+003A (:), then:
 
       <ol>
        <li>