fix: pre-allocate slot zero in every compiled chunk to fix REPL local variable declarations

The compiler now always reserves slot zero for either the closure or method
receiver, eliminating a corner case where top-level REPL code lacked the
pre-allocated slot that function/method bodies expect. This ensures local
variables declared in REPL loops (e.g. `for (i in 1..2)`) get correct slot
indexes.

Additionally, inlines `wrenResetFiber` into `wrenNewFiber` and stores the
imported module's closure on the stack before invoking it, preventing a GC
from collecting the closure during import.

test/language/variable/too_many_locals.wren

@@ -1,6 +1,6 @@
 {
-  var a0 = "value"
-  var a1 = a0
+  // Slot zero is always taken to hold the closure or receiver.
+  var a1 = "value"
   var a2 = a1
   var a3 = a2
   var a4 = a3