js: restore deep copy behavior for Mat.clone()

Emscripten 3.1.71+ introduced ClassHandle.clone() which performs a shallow copy.
This shadowed the original OpenCV clone() method which was intended for deep copying.

This patch:
1. Hooks into onRuntimeInitialized in helpers.js
2. Overrides cv.Mat.prototype.clone to point to mat_clone (deep copy)
3. Updates JS tests to use clone() to verify the fix

modules/js/test/test_mat.js

@@ -173,7 +173,7 @@ QUnit.test('test_mat_creation', function(assert) {
     //   clone
     {
         let mat = cv.Mat.ones(5, 5, cv.CV_8UC1);
-        let mat2 = mat.mat_clone();
+        let mat2 = mat.clone();
 
         assert.equal(mat.channels, mat2.channels);
         assert.equal(mat.size().height, mat2.size().height);