Fix class name of the read marker on MessagePanel (#10745)

* Optimize mx_RoomView_myReadMarker

- Replace the compound selector with a type selector
- Nest the type selector in "mx_RoomView_myReadMarker_container"

* Conform the style rules to the naming policy

Including "container" in a class name is redundant as our naming policy tells that a string followed by an underscore (_) represents a container if it contains an element.

* Run prettier

* Replace `mx_RoomView_myReadMarker hr` with `mx_RoomView_myReadMarker`

- Remove a unit
- Hide hr's parent (mx_RoomView_myReadMarker) on Percy snapshots

* Change the class name from `mx_RoomView_myReadMarker` to `mx_MessagePanel_myReadMarker`

The read marker is specified on MessagePanel.tsx, and should be named so to avoid confusion.

- Create _MessagePanel.pcss for the style rules of MessagePanel

* Update permalinks.spec.ts

res/css/structures/_MessagePanel.pcss

@@ -0,0 +1,35 @@
+/*
+Copyright 2023 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.
+*/
+
+.mx_MessagePanel_myReadMarker {
+    height: 0;
+    margin: 0;
+    padding: 0;
+    border: 0;
+
+    hr {
+        border-top: solid 1px $accent;
+        border-bottom: solid 1px $accent;
+        margin-top: 0;
+        position: relative;
+        top: -1px;
+        z-index: 1;
+        will-change: width;
+        transition: width 400ms easeinsine 1s, opacity 400ms easeinsine 1s;
+        width: 99%;
+        opacity: 1;
+    }
+}