Eliminate redundant CSS module script fetch in workers

Currently the check that disallows creation of CSS module scripts in worker contexts only happens after the fetch for that import is performed. This change moves the check to where we check that the assertion type is a valid value. By doing so, failing that check prevents the fetch.

Closes whatwg#7047.

source

@@ -90958,9 +90958,9 @@ document.querySelector("button").addEventListener("click", bound);
    type</var> be <var>entry</var>.[[Value]]. Otherwise let <var>module type</var> be "<code
    data-x="">javascript</code>".</p></li>
 
-   <li><p>If <var>module type</var> is not "<code data-x="">javascript</code>", "<code
-   data-x="">css</code>", or "<code data-x="">json</code>", then asynchronously complete this
-   algorithm with null, and return.</p></li>
+   <li><p>If the result of running the <span>module type allowed</span> steps given <var>module
+   type</var> and <var>settings object</var> is false, then asynchronously complete this algorithm
+   with null, and return.</p></li>
 
    <li><p><span>Fetch a single module script</span> given <var>url</var>, <var>settings
    object</var>, "<code data-x="">script</code>", <var>options</var>, <var>settings object</var>,
@@ -91437,8 +91437,8 @@ document.querySelector("button").addEventListener("click", bound);
     </ol>
    </li>
 
-   <li><p>Assert: <var>module type</var> is "<code data-x="">javascript</code>", "<code
-   data-x="">css</code>", or "<code data-x="">json</code>". Otherwise we would not have reached
+   <li><p>Assert: the result of running the <span>module type allowed</span> steps given <var>module
+   type</var> and <var>module map settings object</var> is true. Otherwise we would not have reached
    this point because a failure would have been raised when inspecting
    <var>moduleRequest</var>.[[Assertions]] in <a
    href="#validate-requested-module-specifiers">create a JavaScript module script</a> or
@@ -91734,9 +91734,8 @@ document.querySelector("button").addEventListener("click", bound);
      data-x="">javascript</code>".</p></li>
 
      <li>
-      <p>If <var>url</var> is failure, or if <var>module type</var> is not "<code
-      data-x="">javascript</code>", "<code data-x="">css</code>", or "<code data-x="">json</code>",
-      then:</p>
+      <p>If <var>url</var> is failure, or if the result of running the <span>module type
+      allowed</span> steps given <var>module type</var> and <var>settings</var> is false, then:</p>
       <ol>
        <li><p>Let <var>error</var> be a new <code>TypeError</code> exception.</p></li>
 
@@ -91764,10 +91763,6 @@ document.querySelector("button").addEventListener("click", bound);
   string <var>source</var> and an <span>environment settings object</span> <var>settings</var>:</p>
 
   <ol>
-   <li><p>If the <code>CSSStyleSheet</code> interface is not <span
-   data-x="idl-exposed">exposed</span> in <var>setting</var>'s <span data-x="environment settings
-   object's Realm">Realm</span>, then return null.</p></li>
-
    <li><p>Let <var>script</var> be a new <span>module script</span> that this algorithm will
    subsequently initialize.</p></li>
 
@@ -91832,6 +91827,21 @@ document.querySelector("button").addEventListener("click", bound);
    <li><p>Return <var>script</var>.</p></li>
   </ol>
 
+  <p>The <dfn>module type allowed</dfn> steps, given a <span>string</span> <var>module type</var>
+  and an <span>environment settings object</span> <var>settings</var>, are as follows:</p>
+
+  <ol>
+   <li><p>If <var>module type</var> is not "<code data-x="">javascript</code>", "<code
+   data-x="">css</code>", or "<code data-x="">json</code>", then return false.</p></li>
+
+   <li><p>If <var>module type</var> is "<code data-x="">css</code>" and the
+   <code>CSSStyleSheet</code> interface is not <span data-x="idl-exposed">exposed</span> in
+   <var>setting</var>'s <span data-x="environment settings object's Realm">Realm</span>, then
+   return false.</p></li>
+
+   <li><p>Return true.</p></li>
+  </ol>
+
   <h5 id="calling-scripts">Calling scripts</h5>
 
   <p>To <dfn export>run a classic script</dfn> given a <span>classic script</span> <var>script</var>