Summary:
Refactor `preserveSelection()` to use a stack-based approach (`preservedCursors` array) instead of a single cursor reference. This allows nested calls to `preserveSelection()` to operate independently while keeping cursor updates synchronized across active contexts.

Problem:
Using a single stored cursor caused issues in nested calls to `preserveSelection()`:

1. **State overwrite:** Inner calls could overwrite or clear the outer cursor.
2. **Stale references:** If an inner function replaced a DOM node, the outer cursor could still point to a removed node and fail on restore.

Solution:
Use an array of cursor subscribers

- **Shared updates:** When calling `remapNode` on a cursor, it iterates over all active subscribers in the stack. This ensures node replacements performed in inner contexts also update outer cursor references.
- **Scoped cleanup:** `restore()` now removes only the corresponding cursor instance from the stack, ensuring proper lifecycle management.

Example:
The key improvement is that outer scopes receive updates performed by inner scopes.

```javascript
// Function A (outer)
function wrapperFunction() {
    const cursor = this.preserveSelection();
    replaceTextWithSpan();
    cursor.restore();
}

// Function B (inner)
function replaceTextWithSpan() {
    const innerCursor = this.preserveSelection();
    const oldNode = document.querySelector('text');
    const newNode = document.createElement('span');
    oldNode.replaceWith(newNode);
    innerCursor.remapNode(oldNode, newNode);
    innerCursor.restore();
}
```
opw-5386862

---
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

Forward-Port-Of: odoo/odoo#238989