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/elements/_ProgressBar.pcss

@@ -0,0 +1,39 @@
+/*
+Copyright 2020, 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+progress.mx_ProgressBar {
+    height: 6px;
+    width: 60px;
+    overflow: hidden;
+    appearance: none;
+    border: none;
+
+    @mixin ProgressBarBorderRadius 6px;
+    @mixin ProgressBarColour $accent;
+    @mixin ProgressBarBgColour $progressbar-bg-color;
+    ::-webkit-progress-value {
+        transition: width 1s;
+    }
+    ::-moz-progress-bar {
+        transition: padding-bottom 1s;
+        padding-bottom: var(--value);
+        transform-origin: 0 0;
+        transform: rotate(-90deg) translateX(-15px);
+        padding-left: 15px;
+
+        height: 0;
+    }
+}