script: Strip javascript URL scheme using Position::AfterScheme rather than Position::BeforePath (#38599)

This makes the initial split match step 2 of
https://html.spec.whatwg.org/multipage/browsing-the-web.html#evaluate-a-javascript%3A-url.

Testing: Covered by WPT tests.
Fixes: #38547

---------

Signed-off-by: lumiscosity <averyrudelphe@gmail.com>

tests/wpt/meta/MANIFEST.json

@@ -835815,7 +835815,7 @@
      ]
     ],
     "javascript-urls.window.js": [
-     "4f617beca17082dc8459acb73bca781900170593",
+     "3e3a53003dde5fae29ced12ce9f2c26c7520a87c",
      [
       "url/javascript-urls.window.html",
       {}

tests/wpt/meta/url/javascript-urls.window.js.ini

@@ -1,4 +0,0 @@
-[javascript-urls.window.html]
-  expected: ERROR
-  [javascript: URL without an opaque path]
-    expected: FAIL

tests/wpt/tests/url/javascript-urls.window.js

@@ -19,6 +19,12 @@
     "property": "shouldNotExistB",
     "expected": undefined
   },
+  {
+    "description": "javascript: URL with extra slashes at the start",
+    "input": "javascript:///globalThis.shouldNotExistC=1",
+    "property": "shouldNotExistC",
+    "expected": undefined
+  },
   {
     "description": "javascript: URL without an opaque path",
     "input": "javascript://host/1%0a//../0/;globalThis.shouldBeOne=1;/%0aglobalThis.shouldBeOne=2;/..///",