feat: replace unified.js ecosystem with custom markdown parser/serializer

Replace the remark/rehype/unified pipeline (~89 transitive packages) with two
custom, zero-dependency files for markdown↔HTML conversion. The unified ecosystem
was overkill for BlockNote's bounded feature set and required 6 custom plugins
to work around its abstractions.

New files:
- markdownToHtml.ts: block tokenizer + inline parser with recursive list handling
- htmlToMarkdown.ts: DOM-based serializer with proper list nesting and table support

Removes 12 direct dependencies: unified, remark-parse, remark-stringify, remark-gfm,
remark-rehype, rehype-parse, rehype-stringify, rehype-remark, rehype-format,
hast-util-from-dom, unist-util-visit, @types/hast

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nperez0111

packages/core/package.json

@@ -107,31 +107,19 @@
     "@tiptap/pm": "^3.13.0",
     "emoji-mart": "^5.6.0",
     "fast-deep-equal": "^3.1.3",
-    "hast-util-from-dom": "^5.0.1",
     "prosemirror-highlight": "^0.15.1",
     "prosemirror-model": "^1.25.4",
     "prosemirror-state": "^1.4.4",
     "prosemirror-tables": "^1.8.3",
     "prosemirror-transform": "^1.11.0",
     "prosemirror-view": "^1.41.4",
-    "rehype-format": "^5.0.1",
-    "rehype-parse": "^9.0.1",
-    "rehype-remark": "^10.0.1",
-    "rehype-stringify": "^10.0.1",
-    "remark-gfm": "^4.0.1",
-    "remark-parse": "^11.0.0",
-    "remark-rehype": "^11.1.2",
-    "remark-stringify": "^11.0.0",
-    "unified": "^11.0.5",
-    "unist-util-visit": "^5.0.0",
     "uuid": "^8.3.2",
     "y-prosemirror": "^1.3.7",
     "y-protocols": "^1.0.6",
     "yjs": "^13.6.27"
   },
   "devDependencies": {
     "@types/emoji-mart": "^3.0.14",
-    "@types/hast": "^3.0.4",
     "@types/uuid": "^8.3.4",
     "eslint": "^8.57.1",
     "jsdom": "^25.0.1",