fix(framework): reliably re-render lists of DOM nodes

[vladitasev]

In the lit-html repeat directive, the key function is defined as follows:
export type KeyFn<T> = (item: T, index: number) => unknown;
It takes 2 arguments: item and index and the usage is as follows:
newKeys[index] = keyFn ? keyFn(item, index) : index;

We pass undefined to our repeat in litVisitor class. Therefore lit-html takes the index as key.
The purpose of this change is to pass the _id, which is unique to every UI5 Web Component, and the index as fallback, so that the algorithm works as before when there are no _ids (f.e. when we don't iterate on arrays of UI5 Web Components).

This will optimize the rerendering of lists of UI5 Web Comopnents and also prevent some bugs, such as the filtering of items in ui5-multi-combobox.

fixes: #1487