First stage of cleaning up the export handling of the slint root component (#2095)

We implicitly export the last component of a .slint file to the generator.
Issue a warning when that happens and suggest to export it explicitly.

internal/compiler/tests/syntax/functions/functions_call.slint

@@ -10,7 +10,7 @@ Comp := Rectangle {
     public function f2() {}
 }
 
-Xxx := Rectangle {
+export Xxx := Rectangle {
     function foo(a: int) -> string { return a; }
 
     comp := Comp {}