Add language feature for extended main convention

- suspend
- parameterless

 #KT-17679 Fixed
 #KT-26574 Fixed

compiler/testData/diagnostics/tests/redeclarations/RedeclarationSuspendMainInMultiFile_before.kt

@@ -0,0 +1,7 @@
+// !LANGUAGE: -ExtendedMainConvention
+
+// FILE: a.kt
+<!CONFLICTING_OVERLOADS!>suspend fun main(<!UNUSED_PARAMETER!>args<!>: Array<String>)<!> {}
+
+// FILE: b.kt
+<!CONFLICTING_OVERLOADS!>suspend fun main(<!UNUSED_PARAMETER!>args<!>: Array<String>)<!> {}