feat: Implement a rich text editor using Lexical

- Added RichTextEditor component with basic formatting features.
- Integrated toolbar with undo/redo, text formatting, and color pickers.
- Created EditorTheme for styling the editor components.
- Added styles for editor and toolbar.
- Introduced DropdownColorPicker for color selection.
- Updated package.json to include Lexical dependencies.
- Created EditorDemo page to showcase the rich text editor.
- Added README documentation for the editor's features and usage.

src/editor/themes/EditorTheme.ts

@@ -0,0 +1,33 @@
+import type { EditorThemeClasses } from 'lexical';
+
+const theme: EditorThemeClasses = {
+  paragraph: 'editor-paragraph',
+  quote: 'editor-quote',
+  heading: {
+    h1: 'editor-heading-h1',
+    h2: 'editor-heading-h2',
+    h3: 'editor-heading-h3',
+    h4: 'editor-heading-h4',
+    h5: 'editor-heading-h5',
+    h6: 'editor-heading-h6',
+  },
+  list: {
+    nested: {
+      listitem: 'editor-nested-listitem',
+    },
+    ol: 'editor-list-ol',
+    ul: 'editor-list-ul',
+    listitem: 'editor-listitem',
+  },
+  text: {
+    bold: 'editor-text-bold',
+    italic: 'editor-text-italic',
+    underline: 'editor-text-underline',
+    strikethrough: 'editor-text-strikethrough',
+    code: 'editor-text-code',
+  },
+  code: 'editor-code',
+  codeHighlight: {},
+};
+
+export default theme;