Custom Writer extensions for Panel plugins

Similar to custom writerMarks and writerNodes, expose a public Panel plugin registration hook such as writerExtensions.

This would allow Panel plugins to register generic Writer/ProseMirror extensions—for example plugins, decorations, input rules or keymaps—without replacing or extending the core k-writer-input component.

Kirby’s Writer already supports extensions internally. Exposing a stable public hook would make this functionality available to Panel plugins in an upgrade-safe way.