Fix agent string

We don’t have a proper solution for analytics yet ([1] is on hold and
has been removed from mobile beta scope), but in a conversation in the
team we decided that we might as well at least keep the `agent` channel
parameter, which will give us information about channel attaches. So,
name it in line with the JS SDK, which uses "chat-js"; i.e. use the name
of the programming language ("ios" is misleading since we might be
running on macOS or tvOS).

[1] ably/specification#201

Sources/AblyChat/Version.swift

@@ -6,6 +6,6 @@ import Ably
 // Version information
 internal let version = "0.1.0"
 
-internal let channelOptionsAgentString = "chat-ios/\(version)"
+internal let channelOptionsAgentString = "chat-swift/\(version)"
 
 internal let defaultChannelParams = ["agent": channelOptionsAgentString]