Add sendable annotation to userInfo closure

swift-foundation recently landed a change (in
swiftlang/swift-foundation#764) which requires `any Sendable` values in
`JSONEncoder.userInfo`. This causes a build failure:
```
JSONRPCConnection.swift:370:50: error: type '(RequestID) -> Optional<any ResponseType.Type>' does not conform to the 'Sendable' protocol
368 |
369 |     // Setup callback for response type.
370 |     decoder.userInfo[.responseTypeCallbackKey] = { (id: RequestID) -> ResponseType.Type? in
    |                                                  |- error: type '(RequestID) -> Optional<any ResponseType.Type>' does not conform to the 'Sendable' protocol
    |                                                  `- note: a function type must be marked '@sendable' to conform to 'Sendable'
371 |       guard let outstanding = self.outstandingRequests[id] else {
372 |         logger.error("Unknown request for \(id, privacy: .public)")
```

Make the closure sendable (the decoding here is already guarded by a
queue).

Author: bnbarham

Sources/LanguageServerProtocolJSONRPC/JSONRPCConnection.swift

@@ -367,7 +367,7 @@ public final class JSONRPCConnection: Connection {
     decoder.userInfo[.messageRegistryKey] = messageRegistry
 
     // Setup callback for response type.
-    decoder.userInfo[.responseTypeCallbackKey] = { (id: RequestID) -> ResponseType.Type? in
+    decoder.userInfo[.responseTypeCallbackKey] = { @Sendable (id: RequestID) -> ResponseType.Type? in
       guard let outstanding = self.outstandingRequests[id] else {
         logger.error("Unknown request for \(id, privacy: .public)")
         return nil