feat: initial rich-text-elements-ui library implementation

TipTap-powered rich text editing library with WYSIWYG editor, markdown editor,
template editor, collaboration support (Yjs), mentions, track changes,
comments, code blocks, and table insertion.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build-for-dev.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Build the library
+npm run build
+
+# Link it locally for development
+cd dist
+npm link
+
+echo "Library built and linked successfully"
+echo "Run 'npm link @sda/rich-text-elements-ui' in your consuming app"