docs: Document Swift/C++ interoperability feature

docs/markdown/snippets/swift_cxx_interoperability.md

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+## Swift/C++ interoperability is now supported
+
+It is now possible to create Swift executables that can link to C++ or
+Objective-C++ libraries. Only specifying a bridging header for the Swift
+target is required.
+
+Swift 5.9 is required to use this feature. Xcode 15 is required if the
+Xcode backend is used.
+
+```meson
+lib = static_library('mylib', 'mylib.cpp')
+exe = executable('prog', 'main.swift', 'mylib.h', link_with: lib)
+```