[FIR] Put new contract syntax under feauture flag

^KT-55171 Fixed

compiler/fir/analysis-tests/testData/resolve/contracts/disabledFeature.kt

@@ -0,0 +1,26 @@
+// LANGUAGE: -ContractSyntaxV2
+// WITH_STDLIB
+
+import kotlin.contracts.*
+
+inline fun <reified T> requreIsInstance(value: Any) contract <!UNSUPPORTED_FEATURE!>[
+    returns() implies (value is T)
+]<!> {
+    if (value !is T) throw IllegalArgumentException()
+}
+
+val Any?.myLength: Int?
+    get() contract <!UNSUPPORTED_FEATURE!>[
+        <!ERROR_IN_CONTRACT_DESCRIPTION!>returnsNotNull() implies (this@length is String)<!>
+    ]<!> = (this as? String)?.length
+
+fun test_1(x: Any) {
+    requreIsInstance<String>(x)
+    x.length
+}
+
+fun test_2(x: Any) {
+    if (x.myLength != null) {
+        x.<!UNRESOLVED_REFERENCE!>length<!>
+    }
+}