refactor: rename ListSeparators enum to ListSeparator

[ilhan007]

Currently we have BreadcrumbsSeparator in singular form, but ListSeparators in plural and tried to align them.
Looking at the two options, I think the enum values represent individual separator type and should remain in the singular form.

BREAKING CHANGE: The ListSeparators (plural form) enum has been renamed to ListSeparator (singular form).
If you previously imported the ListSeparators:

import ListSeparators from "@ui5/webcomponents/dist/types/ListSeparators.js";
import type ListSeparators from "@ui5/webcomponents/dist/types/ListSeparators.js";

Now, you must import the ListSeparator enumeration as follows:

import ListSeparator from "@ui5/webcomponents/dist/types/ListSeparator.js";
import type ListSeparator from "@ui5/webcomponents/dist/types/ListSeparator.js";

Related to: #8461