Fix #354 by not exporting extern resources

[b-studios]

The problem underlying the error

ReferenceError: my_resource_1396 is not defined

observed by @phischu in #354 was that extern resources have been "exported" in the separate-compilation-based Javascript backends. That is, JS code like

module.export = {
  // ...
  my_resource_1396: my_resource_1396
}

was generated, but extern resources don't have a term-level representation so the right-hand side of my_resource_1396 was unbound.

Solution Attempt

First attempt to solve #354 the problem was

  def shouldExport(sym: Symbol)(using D: DeclarationContext): Boolean = sym match {
    // do not export fields, since they are no defined functions
    case fld if D.findField(fld).isDefined => false
    // do not export effect operations, since they are translated to field selection as well.
    case op if D.findProperty(op).isDefined => false
+   // do not export extern resources, since they do not exist on the term-level
+   case r: symbols.ExternResource => false
    // all others are fine
    case _ => true
  }

However, after core we should not match on symbols anymore; so this is not a good solution, since it is not stable under renaming.

In particular, we currently do not include extern resources in the core representation:

effekt/effekt/shared/src/main/scala/effekt/core/Transformer.scala

Lines 111 to 112 in 3aa78e8

	// For now we forget about all of the following definitions in core:
	case d: source.Def.Extern => Nil

Same holds for ExternType, ExternResource, ExternInterface

Chosen Solution

Instead, now I do not use module.exports on the symbol, anymore. We should try to avoid this after going to core, anyways.

This also helped simplifying the separate compilation aspect of JS a bit (3c26361) since now we do not need to filter twice. I am a little bit afraid that this might introduce a regression, but checked that the tests actually caught this before performing the refactoring.