[Swift Export] KT-65008: support simple top-level properties

Merge-request: KT-MR-14131
Merged-by: Gleb Lukianets <Gleb.Lukianets@jetbrains.com>

native/swift/swift-export-standalone/testData/variables/input_root/main.kt

@@ -0,0 +1,9 @@
+package namespace.main
+
+import namespace.*
+
+val foo: Int get() = 10
+
+var bar: Int = 20
+
+fun foobar(param: Int): Int = foo + bar + param + baz

native/swift/swift-export-standalone/testData/variables/input_root/no_package.kt

@@ -0,0 +1 @@
+fun baz: Int = 30