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

cypress/e2e/right-panel/file-panel.spec.ts

@@ -174,7 +174,7 @@ describe("FilePanel", () => {
             // FIXME: hide mx_SeekBar because flaky - see https://github.com/vector-im/element-web/issues/24897
             //   Remove this once https://github.com/vector-im/element-web/issues/24898 is fixed.
             const percyCSS =
-                ".mx_MessageTimestamp, .mx_RoomView_myReadMarker, .mx_SeekBar { visibility: hidden !important; }";
+                ".mx_MessageTimestamp, .mx_MessagePanel_myReadMarker, .mx_SeekBar { visibility: hidden !important; }";
 
             // Take a snapshot of file tiles list on FilePanel
             cy.get(".mx_FilePanel .mx_RoomView_MessageList").percySnapshotElement("File tiles list on FilePanel", {