Change state to be a tagged-union like type

By turning state from simple string literals to full objects, we can add
annotations that are specific to one state.

Use case: When retrying connections, we could just add a
connectionAttempt?: number; property, but then we would have to check if
it is undefined or not, and if we are not reconnecting currently and
connectionAttempt? is set, does that mean the Sarus client entered an
invalid state?

By guaranteeing certain properties to only ever be present when the
state property has the right kind, we can avoid a whole class of bugs
just by relying on the type system.

src/index.ts

@@ -134,10 +134,25 @@ export default class Sarus {
    *
    * connect(), disconnect() are generally called by the user
    *
-   * this.reconnect() is called internally when automatic reconnection is
-   * enabled, but can also be called by the user
+   * When disconnected by the WebSocket itself (i.e., this.ws.onclose),
+   * this.reconnect() is called automatically if reconnection is enabled.
+   * this.reconnect() can also be called by the user, for example if
+   * this.disconnect() was purposefully called and reconnection is desired.
+   *
+   * The current state is specified by the 'kind' property of state
+   * Each state can have additional data contained in properties other than
+   * 'kind'. Those properties might be unique to one state, or contained in
+   * several states. To access a property, it might be necessary to narrow down
+   * the 'kind' of state.
+   *
+   * The initial state is connecting, as a Sarus client tries to connect right
+   * after the constructor wraps up.
    */
-  state: "connecting" | "connected" | "disconnected" | "closed" = "connecting";
+  state:
+    | { kind: "connecting" }
+    | { kind: "connected" }
+    | { kind: "disconnected" }
+    | { kind: "closed" } = { kind: "connecting" };
 
   constructor(props: SarusClassParams) {
     // Extract the properties that are passed to the class