Add support for edge-distances:endpoints

[maxkfranz]

Cross-references to related issues.

Associated issues:

• edge-distances: endpoints #3157

Notes re. the content of the pull request.

• This change allows bundled bezier edges, unbundled bezier edges, and segment edges to use edge-distances: endpoints.
• The documentation has been updated accordingly.
• The change is minimally invasive by making a new function for the adjustment.
  • The adjustment alters the midpoint and the inverted normal vector used for middle-point calculations (e.g. bezier control points).
  • Limitation: The adjustment must happen at the middle-point phase, rather than the endpoint phase. The endpoint phase happens after the middle point phase, because many edges have automatic endpoints based on the middle points. This means that the endpoint calculations would be duplicated for edge-distances: endpoints edges. In future, this could be optimised if needed.
• Limitation: For a bundled bezier, this implementation assumes that all beziers in the bundle have the same endpoint. [Using different endpoints within a bundle] doesn't seem like a valid use case, anyway.

This PR allows for edges like this, even when the nodes vary in width:

Checklist

Author:

• The proper base branch has been selected. New features go on unstable. Bug-fix patches can go on either unstable or master.
• N/A -- Automated tests have been included in this pull request, if possible, for the new feature(s) or bug fix. Check this box if tests are not pragmatically possible (e.g. rendering features could include screenshots or videos instead of automated tests).
• The associated GitHub issues are included (above).
• Notes have been included (above).

Reviewers:

• All automated checks are passing (green check next to latest commit).
• At least one reviewer has signed off on the pull request.
• For bug fixes: Just after this pull request is merged, it should be applied to both the master branch and the unstable branch. Normally, this just requires cherry-picking the corresponding merge commit from master to unstable -- or vice versa.

[maxkfranz]

@chrtannus, let me know if you notice any issues with this so we can address them prior to merging.

[stale]

This issue has been automatically marked as stale, because it has not had activity within the past 14 days. It will be closed if no further activity occurs within the next 7 days. If a feature request is important to you, please consider making a pull request. Thank you for your contributions.

[chrtannus]

I haven't found any issues, it looks great now.
Here are some tests:

[maxkfranz]

Thanks. Merging