`#SendingRisksDataRace` errors calling async transpiled bridged `@MainActor` methods in a `Task`

[dfabulich]

• Fix Fuse skip-authentication-services#12

Skip Fuse gets #SendingRisksDataRace errors calling async transpiled bridged methods in a Task. (The bridge isn't @MainActor or Sendable.)

/Users/dfabulich/git/skiphack/skipapp-showcase-fuse/Sources/ShowcaseFuse/WebAuthenticationSessionPlayground.swift:89:71: error: sending 'self.webAuthenticationSession' risks causing data races [#SendingRisksDataRace]
 87 |                 let urlWithToken: URL
 88 |                 if #available(iOS 17.4, *) {
 89 |                     urlWithToken = try await webAuthenticationSession.authenticate(
    |                                                                       |- error: sending 'self.webAuthenticationSession' risks causing data races [#SendingRisksDataRace]
    |                                                                       `- note: sending main actor-isolated 'self.webAuthenticationSession' to nonisolated instance method 'authenticate(using:callback:preferredBrowserSession:additionalHeaderFields:)' risks causing data races between nonisolated and main actor-isolated uses
 90 |                         using: URL(string: "https://push.skip.tools/webauth-demo/?scheme=org.appfair.app.showcase")!,
 91 |                         callback: .customScheme("org.appfair.app.showcase"),

/Users/dfabulich/git/skiphack/skipapp-showcase-fuse/Sources/ShowcaseFuse/WebAuthenticationSessionPlayground.swift:96:71: error: sending 'self.webAuthenticationSession' risks causing data races [#SendingRisksDataRace]
 94 |                     )
 95 |                 } else {
 96 |                     urlWithToken = try await webAuthenticationSession.authenticate(
    |                                                                       |- error: sending 'self.webAuthenticationSession' risks causing data races [#SendingRisksDataRace]
    |                                                                       `- note: sending main actor-isolated 'self.webAuthenticationSession' to nonisolated instance method 'authenticate(using:callbackURLScheme:preferredBrowserSession:)' risks causing data races between nonisolated and main actor-isolated uses
 97 |                         using: URL(string: "https://push.skip.tools/webauth-demo/?scheme=org.appfair.app.showcase")!,
 98 |                         callbackURLScheme: "org.appfair.app.showcase",

[#SendingRisksDataRace]: <https://docs.swift.org/compiler/documentation/diagnostics/sending-risks-data-race>

The workaround I used was to add a Sendable extension in an #if SKIP_BRIDGE block.

extension WebAuthenticationSession: @unchecked Sendable {}

… but that adds a warning to the build, "warning: When bridging API from a transpiled module, place all code within a #if !SKIP_BRIDGE condition"