Rename PostCSS files to .pcss (#9013)

* Rename PostCSS files to `.pcss`

* Make Stylelint happy

* Delint

* Rename new files too

* delint

* Fix bad comment placement

res/css/views/auth/_AuthBody.pcss

@@ -52,8 +52,8 @@ limitations under the License.
         @mixin mx_Dialog_link;
     }
 
-    input[type=text],
-    input[type=password] {
+    input[type="text"],
+    input[type="password"] {
         color: $authpage-primary-color;
     }