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>

.gitignore

@@ -0,0 +1,21 @@
+# Dependencies
+node_modules/
+
+# Build output
+dist/
+
+# Angular cache
+.angular/
+
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Misc
+*.log