feat: add module loading with String.import_ and embedder-provided loadModuleFn

Implement initial module system allowing Wren code to import other modules via a temporary `String.import_` method. The embedder must supply a `WrenLoadModuleFn` callback that returns source code for a given module name; the VM caches loaded modules and returns a fiber to execute the module body. Add `wrenImportModule` to the VM API, wire `loadModuleFn` into `WrenConfiguration`, and update the CLI interpreter to resolve imports relative to the entry script's directory. Include test cases for importing multiple variables, shared imports across modules, and verifying that variable bindings are independent across import sites.

test/module/change_imported_value/change_imported_value.wren

@@ -0,0 +1,14 @@
+var Module = "module.wren".import_("Module")
+var Other = "module.wren".import_("Other")
+
+IO.print(Module) // expect: before
+
+// Reassigning the variable in the other module does not affect this one's
+// binding.
+Other.change
+IO.print(Module) // expect: before
+
+// But it does change there.
+Other.show // expect: after
+
+// TODO: Cyclic import.

test/module/change_imported_value/module.wren

@@ -0,0 +1,12 @@
+// nontest
+var Module = "before"
+
+class Other {
+  static change {
+    Module = "after"
+  }
+
+  static show {
+    IO.print(Module)
+  }
+}

test/module/multiple_variables/module.wren

@@ -0,0 +1,8 @@
+// nontest
+var Module1 = "from module one"
+var Module2 = "from module two"
+var Module3 = "from module three"
+var Module4 = "from module four"
+var Module5 = "from module five"
+
+IO.print("ran module")

test/module/multiple_variables/multiple_variables.wren

@@ -0,0 +1,14 @@
+var Module1 = "module.wren".import_("Module1")
+var Module2 = "module.wren".import_("Module2")
+var Module3 = "module.wren".import_("Module3")
+var Module4 = "module.wren".import_("Module4")
+var Module5 = "module.wren".import_("Module5")
+
+// Only execute module body once:
+// expect: ran module
+
+IO.print(Module1) // expect: from module one
+IO.print(Module2) // expect: from module two
+IO.print(Module3) // expect: from module three
+IO.print(Module4) // expect: from module four
+IO.print(Module5) // expect: from module five

test/module/shared_import/a.wren

@@ -0,0 +1,5 @@
+// nontest
+IO.print("a")
+var Shared = "shared.wren".import_("Shared")
+var A = "a " + Shared
+IO.print("a done")

test/module/shared_import/b.wren

@@ -0,0 +1,5 @@
+// nontest
+IO.print("b")
+var Shared = "shared.wren".import_("Shared")
+var B = "b " + Shared
+IO.print("b done")

test/module/shared_import/shared.wren

@@ -0,0 +1,3 @@
+// nontest
+IO.print("shared")
+var Shared = "shared"

test/module/shared_import/shared_import.wren

@@ -0,0 +1,12 @@
+var A = "a.wren".import_("A")
+var B = "b.wren".import_("B")
+
+// Shared module should only run once:
+// expect: a
+// expect: shared
+// expect: a done
+// expect: b
+// expect: b done
+
+IO.print(A) // expect: a shared
+IO.print(B) // expect: b shared

test/module/simple_import/module.wren

@@ -0,0 +1,3 @@
+// nontest
+var Module = "from module"
+IO.print("ran module")

test/module/simple_import/simple_import.wren

@@ -0,0 +1,4 @@
+var Module = "module.wren".import_("Module")
+// expect: ran module
+
+IO.print(Module) // expect: from module

test/module/unknown_module/unknown_module.wren

@@ -0,0 +1 @@
+var DoesNotExist = "does_not_exist.wren".import_("DoesNotExist") // expect runtime error: Could not find module 'does_not_exist.wren'.

test/module/unknown_variable/module.wren

@@ -0,0 +1,3 @@
+// nontest
+var Module = "from module"
+IO.print("ran module")

test/module/unknown_variable/unknown_variable.wren

@@ -0,0 +1,3 @@
+// Should execute the module:
+// expect: ran module
+var DoesNotExist = "module.wren".import_("DoesNotExist") // expect runtime error: Could not find a variable named 'DoesNotExist' in module 'module.wren'.