editor: fix circular import that dropped PubScript nodes from schema

The TipTap node definitions imported their node-name constants from
schema.ts, while schema.ts imported the nodes to build the schema. This
circular dependency evaluated the nodes before the constants were
assigned, leaving sceneBreak/footnote/footnoteArea/characterMention with
name 'undefined' — so they never registered in PubScriptSchema. As a
result <hr class="scene-break"> parsed as StarterKit's horizontalRule
instead of a scene break.

- Move node-name constants, groups, MentionAttrs, and colorAttribute into
  a new constants.ts module that both schema.ts and the node files import
  from, breaking the cycle.
- Remove the redundant separate History extension (already in StarterKit),
  which also silenced a TipTap 'Duplicate extension names: undoRedo' warn.
- Drop unused TipTap extension and prosemirror-* deps (all access goes
  through @tiptap/pm).

packages/editor/tsconfig.json

@@ -0,0 +1,8 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "jsx": "react-jsx",
+    "types": ["node"]
+  },
+  "include": ["src/**/*.ts", "src/**/*.tsx"]
+}