Add macOS support warning in steamworks section

src/locales/en.json

@@ -561,6 +561,10 @@
       "string": "Steamworks Extension",
       "developer_comment": "Title of section that appears when using the Steamworks extension"
     },
+    "steamworksMacWarning": {
+      "string": "Warning: macOS games published on Steam need to be notarized by Apple, which the packager doesn't support. You can still test your game on a Mac, but you won't be able to publish it for macOS yet.",
+      "developer_comment": "When using the Steamworks extension, warning that appears whenever macOS support is used or mentioned."
+    },
     "steamworksDocumentation": {
       "string": "See the extension's documentation for more details.",
       "developer_comment": "Link to more documentation about the Steamworks extension"

src/p4/PackagerOptions.svelte

@@ -1035,11 +1035,21 @@
           <option value="error">{$_('options.steamworksError')}</option>
         </select>
       </label>
+
+      {#if $options.target === 'electron-mac'}
+        <p class="warning">
+          {$_('options.steamworksMacWarning')}
+        </p>
+      {/if}
     {:else}
       <p>{$_('options.steamworksUnavailable')}</p>
       <ul>
         <li>{$_('options.application-win64').replace('{type}', 'Electron')}</li>
-        <li>{$_('options.application-mac').replace('{type}', 'Electron')}</li>
+        <li>
+          {$_('options.application-mac').replace('{type}', 'Electron')}
+          <br>
+          {$_('options.steamworksMacWarning')}
+        </li>
         <li>{$_('options.application-linux64').replace('{type}', 'Electron')}</li>
       </ul>
     {/if}