anchor-scroll.js throws a TypeError when the anchor_scroll library is loaded while anchor scroll is disabled

Problem/Motivation

bs_lib_page_attachments() attaches drupalSettings.bs_lib.anchor_scroll
only when anchor_scroll.enable is set in bs_lib.settings. However, the
bs_lib/anchor_scroll library can also reach the page as a dependency of other
libraries (for example a theme TOC library that needs BSLib.getFixedElementsOffset()).
In that case js/anchor-scroll.js loads without its settings and throws:

• window.BSLib.getFixedElementsOffset() reads
  drupalSettings.bs_lib.anchor_scroll unguarded and throws

  TypeError: Cannot read
    properties of undefined (reading 'anchor_scroll')

  for every caller.

• Drupal.behaviors.BSLibAnchorScroll.attach() reads
  drupalSettings.bs_lib.anchor_scroll.exclude_links unguarded and throws on every
  behaviors attach, which can break other behaviors on the page.

Steps to reproduce

1. Set anchor_scroll.enable: false in bs_lib.settings.
2. Add bs_lib/anchor_scroll as a dependency to any custom library and attach it to a
   page.
3. Load the page: Drupal.behaviors.BSLibAnchorScroll throws a TypeError
   on attach, and any call to BSLib.getFixedElementsOffset() throws as well.

Proposed resolution

Do not rely on the settings being present:

• getFixedElementsOffset() returns 0 when
  drupalSettings.bs_lib.anchor_scroll is not available. This also keeps the
  location.hash auto-scroll path safe, since it calls through
  calculateScrollOffset().
• Drupal.behaviors.BSLibAnchorScroll.attach() returns early when the settings are
  not available - without them there is no anchor scroll configuration, so there is nothing to
  attach.