Dialog: Provide hints for the sender name

The dialog form has a simple text input for the sender's name.
This can lead to typos or mistakes if the GM wants to send a message
using the same sender name as a previous message.

Add a new function that goes through all the players' histories and
compiles a list of all unique senders, which we can then use to build
a list of hints that will be used to show suggestions when the GM fills
the sender name in the form.

This is currently quite ineficcient as it re-does all the work for every
single dialog.
A better way would be to cache it or store it explicitly somewhere when
we send a message, but this would require a way for the GM to edit it.

scripts/utils.mjs

@@ -50,3 +50,28 @@ export async function create_history_journal(user) {
         return user.setFlag(module_id, history_flag, journal.id).then(() => journal);
     });
 }
+
+/**
+ * Go through all the user's histories to build a list of existing senders,
+ * so that they can be displayed or re-used.
+ * The final list is sorted and does not contain duplicates, as one sender
+ * can send messages to multiple players at once.
+ * @returns {String[]} A sorted array of the sender names, if any
+ */
+export function get_existing_senders() {
+    /** @type {String[]} */
+    let senders = []
+    for (let user of game.users.players) {
+        /** @type {JournalEntry} */
+        let history = game.journal.get(user.getFlag(module_id, history_flag))
+        if (!history) {
+            continue;
+        }
+        for (let page of history.pages) {
+            if (!senders.includes(page.name)) {
+                senders.push(page.name);
+            }
+        }
+    }
+    return senders.sort();
+}