Add support for Diff split/unified field layouts

Problem/Motivation

The Diff module can display differences in the rendered display of an entity, and/or the differences in individual fields on the split/unified fields layouts (as configured at /admin/config/content/diff/fields).

The only current diff option for weight fields is "Don't compare" because there's no diff builder plugin that supports weight fields. Since it's often likely that weight fields would not appear in the rendered entity display, there's no easy way to know when/how a weight field has changed.

Steps to reproduce

Visit /admin/config/content/diff/fields and note that the only option for weight fields is "Don't compare."

Proposed resolution

Diff's CoreFieldBuilder plugin already supports integers, so it should work well for weight fields. Implement hook_field_diff_builder_info_alter() to add the weight field type id to core_field_diff_builder's field_types.