Gradual migration of operator 'mod' to 'rem'

- Introduce new 'rem' operator convention
 - Prefer 'rem()' to 'mod()' when both are available, even if mod() is a
   member, and rem() -- an extension
 - Place operator 'rem' under the language feature

compiler/testData/diagnostics/tests/operatorRem/prefereRemAsExtensionOverMemberMod.txt

@@ -0,0 +1,13 @@
+package
+
+public fun foo(): kotlin.Unit
+public fun takeInt(/*0*/ x: kotlin.Int): kotlin.Unit
+public operator fun Foo.rem(/*0*/ x: kotlin.Int): kotlin.Int
+
+public final class Foo {
+    public constructor Foo()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final operator fun mod(/*0*/ x: kotlin.Int): Foo
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}