Use the more optimal overload for copying val (#24694)

I found by accident (while stepping via the debugger) that for `val(v)`
where `v` is `val&` C++ chooses the `T&&` overload as more precise one
instead of `const val&`.

The runtime behaviour of the two is equivalent, so I'm not sure it can
be easily tested, but forwarding to the const overload would certainly
be a bit more optimal.

Author: RReverser

system/include/emscripten/val.h

@@ -368,6 +368,11 @@ class EMBIND_VISIBILITY_DEFAULT val {
     }
   }
 
+  // Add an explicit overload for `val&` as well.
+  // Without it, C++ will try to use the `T&&` constructor instead of the more
+  // efficient `val(const val&)` when trying to copy a `val` instance.
+  val(val& v) : val(static_cast<const val&>(v)) {}
+
   ~val() {
     if (uses_ref_count()) {
       internal::_emval_decref(as_handle());