Add logging in and requesting saves list

Note: eventually I'll move the types into comments so this can be a plain .js file anyone can use

galaxy.ts

@@ -0,0 +1,95 @@
+// TODO remove types and put them in comments so this can be a .js file
+
+export function initGalaxy({
+    acceptedOrigins,
+    supportsSaving,
+    supportsSaveManager,
+    ignoreApiVersion,
+    onLoggedInChanged
+}: {
+    acceptedOrigins?: string[];
+    supportsSaving?: boolean;
+    supportsSaveManager?: boolean;
+    ignoreApiVersion?: boolean;
+    onLoggedInChanged?: (loggedIn: boolean) => void;
+}) {
+    return new Promise((accept, reject) => {
+        acceptedOrigins = acceptedOrigins ?? ["https://galaxy.click"];
+        if (acceptedOrigins.includes(window.origin)) {
+            // Callbacks to resolve promises
+            let saveListAccept: (value: SaveListResponse["list"]) => void,
+                saveListReject: (reason?: string) => void;
+
+            const galaxy = {
+                acceptedOrigins,
+                supportsSaving,
+                supportsSaveManager,
+                ignoreApiVersion,
+                onLoggedInChanged,
+                origin: window.origin,
+                apiVersion: 0,
+                loggedIn: false,
+                postMessage: function (
+                    message: SupportsAction | SaveListAction | SaveAction | LoadAction
+                ) {
+                    window.top?.postMessage(message, galaxy.origin);
+                },
+                getSaveList: function () {
+                    if (saveListAccept != null || saveListReject != null) {
+                        console.warn("save_list action already in progress. Ignoring...");
+                        return;
+                    }
+                    galaxy.postMessage({
+                        action: "save_list"
+                    });
+                    return new Promise<SaveListResponse["list"]>((accept, reject) => {
+                        saveListAccept = accept;
+                        saveListReject = reject;
+                    });
+                }
+            };
+
+            window.addEventListener("message", e => {
+                if (e.origin === galaxy.origin) {
+                    console.log("Received message from Galaxy", e.data);
+                    const data = e.data as GalaxyResponse;
+
+                    switch (data.type) {
+                        case "info": {
+                            const { galaxy: isGalaxy, api_version, logged_in } = data;
+                            // Ignoring isGalaxy check in case other accepted origins send it as false
+                            if (api_version !== 1 && galaxy.ignoreApiVersion !== true) {
+                                reject(`API version not recognized: ${api_version}`);
+                            } else {
+                                // Info responses may be sent again if the information gets updated
+                                // Specifically, we care if logged_in gets changed
+                                // We can use the api_version to determine if this is the first
+                                // info response or a new one.
+                                const firstInfoResponse = galaxy.apiVersion === 0;
+                                galaxy.apiVersion = api_version;
+                                galaxy.loggedIn = logged_in;
+                                galaxy.origin = e.origin;
+                                if (firstInfoResponse) {
+                                    accept(galaxy);
+                                } else {
+                                    galaxy.onLoggedInChanged?.(logged_in);
+                                }
+                            }
+                            break;
+                        }
+                        case "save_list": {
+                            const { list, error, message } = data;
+                            if (error) {
+                                saveListReject(message);
+                            } else {
+                                saveListAccept(list);
+                            }
+                        }
+                    }
+                }
+            });
+        } else {
+            reject(`Project is not running on an accepted origin: ${window.origin}`);
+        }
+    });
+}