Conditionally conform to Combine.TopLevelDecoder (CoreOffice#132)

This enables use of `XMLDecoder` with Combine's `decode(_:decoder:)` operator by conditionally conforming to `TopLevelDecoder` when Combine is available.

I explored doing the same for `TopLevelEncoder`, but `XMLEncoder` currently requires `rootKey:` to be specified at encoding time, so it's not possible to provide an implementation of `encode(_:)` without some design around how to dynamically determine the root key to use.

* Conditionally conform to Combine.TopLevelDecoder
* Add test and documentation for Combine integration
* Remove macCatalyst from CombineTests
This shouldn't be necessary because all versions of Mac Catalyst should
have a version of Combine available.
* Add CombineTests to the Xcode project
* Disable CombineTests on macOS

Author: sharplet

README.md

@@ -268,10 +268,16 @@ extension IntOrString: Codable {
 This is described in more details in PR [\#119](https://github.com/MaxDesiatov/XMLCoder/pull/119) 
 by [@jsbean](https://github.com/jsbean) and [@bwetherfield](https://github.com/bwetherfield).
 
+<<<<<<< HEAD
 ### Integrating with [Combine](https://developer.apple.com/documentation/combine)
 
 Starting with XMLCoder [version 0.9](https://github.com/MaxDesiatov/XMLCoder/releases/tag/0.9.0), 
 when Apple's Combine framework is available, `XMLDecoder` conforms to the
+=======
+## Integrating with [Combine](https://developer.apple.com/documentation/combine)
+
+When Apple's Combine framework is available, `XMLDecoder` conforms to the
+>>>>>>> Conditionally conform to Combine.TopLevelDecoder (#132)
 `TopLevelDecoder` protocol, which allows it to be used with the
 `decode(type:decoder:)` operator: