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/permalinks/permalinks.spec.ts

@@ -132,7 +132,7 @@ describe("permalinks", () => {
         cy.get(".mx_GenericEventListSummary").invoke("remove");
 
         // Disabled because flaky - see https://github.com/vector-im/element-web/issues/25283
-        //const percyCSS = ".mx_MessageTimestamp, .mx_RoomView_myReadMarker { visibility: hidden !important; }";
+        //const percyCSS = ".mx_MessageTimestamp, .mx_MessagePanel_myReadMarker { visibility: hidden !important; }";
         //cy.get(".mx_RoomView_timeline").percySnapshotElement("Permalink rendering", { percyCSS });
     });
 });