.user.css in virtual style urls

benefits:
* styles are listed directly under `Stylus` in devtools source file tree
* easy to differentiate from site's css

background/style-via-webrequest.js

@@ -51,7 +51,7 @@
     }
     if (CHROME) {
       chrome.webRequest.onBeforeRequest.addListener(openNamedStyle, {
-        urls: [URLS.ownOrigin + 'virtual/*.css'],
+        urls: [URLS.ownOrigin + '*.user.css'],
         types: ['main_frame'],
       }, ['blocking']);
     }

content/style-injector.js

@@ -150,7 +150,7 @@ window.StyleInjector = window.INJECTED === 1 ? window.StyleInjector : ({
     if (exposeStyleName && name) {
       el.dataset.name = name;
       name = encodeURIComponent(name.replace(/[#%/@:']/g, toSafeChar));
-      code += `\n/*# sourceURL=${chrome.runtime.getURL('')}virtual/${name}.css#${id} */`;
+      code += `\n/*# sourceURL=${chrome.runtime.getURL(name)}.user.css#${id} */`;
     }
     el.type = 'text/css';
     // SVG className is not a string, but an instance of SVGAnimatedString