Add language feature for suspend functions in fun interfaces

Since in 1.5 JVM_IR is enabled by default and supports suspend functions
in fun interfaces, it is safe to enable the feature by default as well.

compiler/testData/diagnostics/tests/functionLiterals/suspend/disabled.fir.kt

@@ -0,0 +1,5 @@
+// LANGUAGE: -SuspendFunctionsInFunInterfaces, +JvmIrEnabledByDefault
+
+fun interface I {
+    suspend fun foo()
+}

compiler/testData/diagnostics/tests/functionLiterals/suspend/disabled.kt

@@ -0,0 +1,5 @@
+// LANGUAGE: -SuspendFunctionsInFunInterfaces, +JvmIrEnabledByDefault
+
+fun interface I {
+    <!FUN_INTERFACE_WITH_SUSPEND_FUNCTION!>suspend<!> fun foo()
+}

compiler/testData/diagnostics/tests/functionLiterals/suspend/disabled.txt

@@ -0,0 +1,8 @@
+package
+
+public fun interface I {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract suspend fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/functionLiterals/suspend/enabled.kt

@@ -0,0 +1,6 @@
+// FIR_IDENTICAL
+// LANGUAGE: +SuspendFunctionsInFunInterfaces, +JvmIrEnabledByDefault
+
+fun interface I {
+    suspend fun foo()
+}

compiler/testData/diagnostics/tests/functionLiterals/suspend/enabled.txt

@@ -0,0 +1,8 @@
+package
+
+public fun interface I {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract suspend fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}