Rename Subscription’s finish method to unsubscribe

I don’t think that this method should exist at all; the intention for
our public API, as described in 20e7f5f, is that the user should not
need to explicitly unsubscribe. (We have #36 for figuring out how to
implement the appropriate cleanup.) However, this method does now exist,
and we’re probably not going to get rid of it before beta, so we agreed
in standup today to at least name it in line with JS.

Sources/AblyChat/Subscription.swift

@@ -72,7 +72,7 @@ public struct Subscription<Element: Sendable>: Sendable, AsyncSequence {
     }
 
     // TODO: https://github.com/ably-labs/ably-chat-swift/issues/36 Revisit how we want to unsubscribe to fulfil CHA-M4b & CHA-ER4b. I think exposing this publicly for all Subscription types is suitable.
-    public func finish() {
+    public func unsubscribe() {
         switch mode {
         case let .default(_, continuation):
             continuation.finish()