Support incremental footnotes block update

Problem/Motivation

The Footnotes block introduced in ✨ Provide a "footnotes" block collecting all definitions present on a page Fixed only generates the footnotes block once, as late as it can, which is enough to work both with and without JS aggregation and big_pipe.

For complete Drupal compatibility with delayed content appearing in further blocks, the footnotes generation and anchors transformation needs to support incremental regeneration.

Steps to reproduce

• Find a way to have a block that loads after G2 Footnotes, AND contains DFN links (e.g. a views block in some cases
• Ensure it triggers attachBehaviors on a new context
• Verify it does not cause a G2Footnotes regeneration.

Proposed resolution

Support the feature.

Remaining tasks

TBD.

User interface changes

This may need some extra state classes on anchors and/or footnotes and/or block.

API changes

None.

Data model changes

None.