Add a preference for the size of vector point anchor/handle manipulators

This isn't a priority and it will add a maintenance burden so this isn't suggested for completion in 2025, but at some later point in time once it's likely to benefit enough users to be worthwhile.

The issue names no files, tests, or entry points, and says only that a preference for vector point anchor/handle manipulator size may be worthwhile later. First clarify the intended scope and where preferences and vector manipulators are implemented; done would require an agreed design and a working size preference.

索引モデルが issue の本文から書いたものです。