Fix {{firstIncludedMessageId}} to not rely on DOM

Up until now, the {{firstIncludedMessageId}} macro relied on searching the DOM for the actual CSS class applied for the context line.
Bad design, as the actual message was maybe not display, with display X messages enabled.

- Use setInContextMessages() that sets the context line on generation to also update a chat metadata field
- Utilize that field inside the macro
- Update docs to clarify that this will only show the mesid that was relevant during last generation

public/scripts/macros.js

@@ -202,13 +202,7 @@ export function getLastMessageId({ exclude_swipe_in_propress = true, filter = nu
  * @returns {number|null} The ID of the first message in the context
  */
 function getFirstIncludedMessageId() {
-    const index = Number(document.querySelector('.lastInContext')?.getAttribute('mesid'));
-
-    if (!isNaN(index) && index >= 0) {
-        return index;
-    }
-
-    return null;
+    return chat_metadata['lastInContextMessageId'];
 }
 
 /**