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.

include/wren.h

@@ -25,8 +25,12 @@ typedef struct WrenVM WrenVM;
 //   [oldSize] will be zero. It should return NULL.
 typedef void* (*WrenReallocateFn)(void* memory, size_t oldSize, size_t newSize);
 
+// A function callable from Wren code, but implemented in C.
 typedef void (*WrenForeignMethodFn)(WrenVM* vm);
 
+// Loads and returns the source code for the module [name].
+typedef char* (*WrenLoadModuleFn)(WrenVM* vm, const char* name);
+
 typedef struct
 {
   // The callback Wren will use to allocate, reallocate, and deallocate memory.
@@ -34,6 +38,23 @@ typedef struct
   // If `NULL`, defaults to a built-in function that uses `realloc` and `free`.
   WrenReallocateFn reallocateFn;
 
+  // The callback Wren uses to load a module.
+  //
+  // Since Wren does not talk directly to the file system, it relies on the
+  // embedder to phyisically locate and read the source code for a module. The
+  // first time an import appears, Wren will call this and pass in the name of
+  // the module being imported. The VM should return the soure code for that
+  // module. Memory for the source should be allocated using [reallocateFn] and
+  // Wren will take ownership over it.
+  //
+  // This will only be called once for any given module name. Wren caches the
+  // result internally so subsequent imports of the same module will use the
+  // previous source and not call this.
+  //
+  // If a module with the given name could not be found by the embedder, it
+  // should return NULL and Wren will report that as a runtime error.
+  WrenLoadModuleFn loadModuleFn;
+
   // The number of bytes Wren will allocate before triggering the first garbage
   // collection.
   //