Harden custom caret plugin for multiple editors

[Codex]

Greptile flagged two risks in the new animated caret: module-level state breaks multiple editors and plugin silently fails if the editor DOM parent is missing.

• Scoped instances: track caret elements per EditorView so setCaretStyle updates all views and cleanup is per-instance.
• Init guard: bail with a warning when no parent container exists instead of silently failing.

Example:

const caretInstances = new Set<{ view: EditorView; el: HTMLElement }>();

export function setCaretStyle(style: CaretStyle) {
  caretStyle = style;
  caretInstances.forEach(({ view, el }) => updateCaret(view, el));
}

view(view) {
  const parent = view.dom.parentElement;
  if (!parent) {
    console.warn("caretPlugin: editor has no parent element; skipping custom caret");
    return {};
  }
  // ...
}