🆔 Introducing Stable Element IDs for PB elemets

[m0nikaza]

Before 5.40 the system was regenerating element IDs every time a page/a block or a template is opened in one of our editors. Now we are putting in place a stable reference ID for each element to allow us to extend the functionalities we offer in the future.
For example, we are planning on using these stable IDs for Translations, but most users will benefit from this change immediately, e.g. using them for triaging issues will make error resolution a lot quicker.

[m0nikaza]

More details on the logic can be found here: #3977