Parser tests: move the contract description blocks tests to a separate folder and add a test for property accessors' contracts

compiler/testData/psi/contracts/FunctionWithMultilineContract.kt

@@ -0,0 +1,13 @@
+fun baz(num: Int?, element: MyElement, block: () -> Unit): Int contract [
+    callsInPlace(block, InvocationKind.EXACTLY_ONCE),
+    returns() implies (num != null),
+    returns() implies (element != null)
+] {
+    require(num != null)
+    require(element != null)
+    block()
+    if (num >= 0) {
+        return 1;
+    }
+    return 0
+}