EWPP-2149: Allow inpage navigation generation to support multiple instances.

[brummbar]

This PR has a different approach than #1086.
The reasoning behind this rework are:

• The above PR forces to specify where the generated items need to be added. If we have a template has very special elements that are rendered and need to act as titles (e.g. span.fw-bold), we need to somehow fetch the ID generated in the inpage content row paragraph so that we can pass it to the element. The idea behind the data-inpage-navigation-source-area was to be able to declare any template as source area, without having to do any extra steps.
• This PR reworks the JS to be more similar to a standalone library/plugin. Aside from the name (which we cannot change if the users are referencing its old methods), all the ECL specific selectors are gone from Drupal.eclInPageNavigation.