Add a utility to the API for inferring a chat message's roll mode (and update the docs)

module/utils/inferRollMode.mjs

@@ -0,0 +1,36 @@
+/**
+ * A helper function to try and infer what roll mode was used when creating a
+ * chat message in case the roll mode was not provided during the createChatMessage
+ * hook for whatever reason.
+ *
+ * **Disclaimer**: This inference is not totally correct. Particularly when inferring
+ * a GM's message, as it won't be able to distinguish between a self-roll and a
+ * private GM roll when it's
+ *
+ * @param {ChatMessage} message The ChatMessage document to infer from
+ * @returns The Foundry-specified roll mode
+ */
+export function inferRollMode(message) {
+	const whisperCount = message.whisper.length;
+	if (whisperCount === 0) {
+		return CONST.DICE_ROLL_MODES.PUBLIC;
+	};
+
+	if (whisperCount === 1 && message.whisper[0] === game.user.id) {
+		return CONST.DICE_ROLL_MODES.SELF;
+	};
+
+	let allGMs = true;
+	for (const userID of message.whisper) {
+		const user = game.users.get(userID);
+		if (!user) { continue };
+		allGMs &&= user.isGM;
+	};
+
+	if (!allGMs) {
+		return CONST.DICE_ROLL_MODES.PUBLIC;
+	};
+	return message.blind
+		? CONST.DICE_ROLL_MODES.BLIND
+		: CONST.DICE_ROLL_MODES.PRIVATE;
+};