Objective-C support in the Swift Package version

[younata]

Resolves https://github.com/Quick/Nimble/issues/989

• Adds Objective-C support for the SwiftPM version of Nimble

There's a lot of follow-on, entirely stemming from the fact that SwiftPM doesn't support mixed-language targets.

• Creates a NimbleSharedTestHelpers internal target, used only for testing Swift and Objective-C. This should not be exported or used by others. This exists only in the SPM version for code-sharing purposes.
• Separates the old Tests/NimbleTests/objc folder out into Tests/NimbleObjectiveCTests/
• Converts a lot of #if SWIFT_PACKAGE to #if canImport(Darwin) (or with the equivalent !'ed versions), because now we do support Objective-C in SwiftPM.
• SwiftPM users w/ objc tests will have to use both @import Nimble; and @import NimbleObjectiveC;
• Capturing objective-c exceptions doesn't appear to be working. Still investigating why.
• Failure messages are slightly different for the SwiftPM version of objective-c.

Todo

• [ ] Write documentation on how to use this.
• [ ] Further investigate what's going on with capturing objective-c exceptions in swiftpm.

[0xpablo]

Hi there, we recently updated nimble to the latest version and it seems this PR broke Swift WASM on macOS (as it's trying to build the NimbleObjectiveC which then fails). I will open a ticket later.