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/noOperatorRemFeature.kt

@@ -0,0 +1,16 @@
+// !LANGUAGE: -OperatorRem
+// !DIAGNOSTICS: -UNUSED_PARAMETER, -UNUSED_VARIABLE
+
+class Foo {
+    <!UNSUPPORTED_FEATURE!>operator<!> fun rem(x: Int): Foo = Foo()
+}
+
+class Bar {
+    <!UNSUPPORTED_FEATURE!>operator<!> fun remAssign(x: Int) {}
+}
+
+fun baz() {
+    val f = Foo() % 1
+    val b = Bar()
+    b %= 1
+}