feat: split core module from main and add implicit core imports for all modules

Extract core module loading into separate `loadIntoCore` function and `getCoreModule` helper, replacing the old `getMainModule`. Implement `loadModule` that automatically imports all core module variables into every newly loaded module, enabling implicit visibility of core types (Bool, Class, Fiber, etc.) across all user modules. Add `wrenImportModule` public API for module loading with circular import detection. Increase `WREN_MAX_TEMP_ROOTS` from 4 to 5 to accommodate additional GC roots during module loading. Add test cases for cyclic imports (a.wren/b.wren) and implicit core imports (implicitly_imports_core.wren/module.wren).

test/module/cyclic_import/b.wren

@@ -0,0 +1,9 @@
+// nontest
+IO.print("start b")
+
+var B = "b value"
+IO.print("b defined ", B)
+var A = "a".import_("A")
+IO.print("b imported ", A)
+
+IO.print("end b")