Make it compatible without extra config on Transformers.js

hi

@OriLib

, this file is need on transformers.js , with it on the repo, it makes is super simple to load it without passing extra params
after
```js
import { AutoModel, AutoProcessor, RawImage } from '@xenova/transformers';

// Load model and processor
const model = await AutoModel.from_pretrained('briaai/RMBG-1.4', { quantized: false });
const processor = await AutoProcessor.from_pretrained('briaai/RMBG-1.4');

```
before

```js
const model = AutoModel.from_pretrained("briaai/RMBG-1.4", {
// Do not require config.json to be present in the repository
config: { model_type: "custom" },
quantized: device === Devices.webgpu ? false : quantized,
device: device,
});
console.log("RUNNING WIHTT", device);

const processor = await AutoProcessor.from_pretrained("briaai/RMBG-1.4", {
// Do not require config.json to be present in the repository
config: {
do_normalize: true,
do_pad: false,
do_rescale: true,
do_resize: true,
image_mean: [0.5, 0.5, 0.5],
feature_extractor_type: "ImageFeatureExtractor",
image_std: [1, 1, 1],
resample: 2,
rescale_factor: 0.00392156862745098,
size: { width: 1024, height: 1024 },
},
});
````

preprocessor_config.json

@@ -0,0 +1,25 @@
+{
+    "config": {
+        "do_normalize": true,
+        "do_pad": false,
+        "do_rescale": true,
+        "do_resize": true,
+        "image_mean": [
+            0.5,
+            0.5,
+            0.5
+        ],
+        "feature_extractor_type": "ImageFeatureExtractor",
+        "image_std": [
+            1,
+            1,
+            1
+        ],
+        "resample": 2,
+        "rescale_factor": 0.00392156862745098,
+        "size": {
+            "width": 1024,
+            "height": 1024
+        }
+    }
+}