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/styles/editor.css

@@ -0,0 +1,148 @@
+.editor-container {
+  margin: 20px auto;
+  border-radius: 8px;
+  max-width: 1100px;
+  color: #000;
+  position: relative;
+  line-height: 1.7;
+  font-weight: 400;
+  text-align: left;
+  border: 1px solid #e0e0e0;
+  background: #fff;
+}
+
+.editor-inner {
+  background: #fff;
+  position: relative;
+  border-bottom-left-radius: 8px;
+  border-bottom-right-radius: 8px;
+}
+
+.editor-input {
+  min-height: 300px;
+  resize: vertical;
+  font-size: 15px;
+  caret-color: rgb(5, 5, 5);
+  position: relative;
+  tab-size: 1;
+  outline: 0;
+  padding: 15px 20px;
+  caret-color: #444;
+}
+
+.editor-placeholder {
+  color: #999;
+  overflow: hidden;
+  position: absolute;
+  text-overflow: ellipsis;
+  top: 15px;
+  left: 20px;
+  font-size: 15px;
+  user-select: none;
+  display: inline-block;
+  pointer-events: none;
+}
+
+.editor-paragraph {
+  margin: 0;
+  margin-bottom: 8px;
+  position: relative;
+}
+
+.editor-paragraph:last-child {
+  margin-bottom: 0;
+}
+
+.editor-heading-h1 {
+  font-size: 2em;
+  font-weight: 700;
+  margin: 0;
+  margin-bottom: 12px;
+  padding: 0;
+}
+
+.editor-heading-h2 {
+  font-size: 1.5em;
+  font-weight: 700;
+  margin: 0;
+  margin-bottom: 10px;
+  padding: 0;
+}
+
+.editor-heading-h3 {
+  font-size: 1.25em;
+  font-weight: 700;
+  margin: 0;
+  margin-bottom: 8px;
+  padding: 0;
+}
+
+.editor-quote {
+  margin: 0;
+  margin-left: 20px;
+  margin-bottom: 10px;
+  font-size: 15px;
+  color: #666;
+  border-left: 4px solid #ccc;
+  padding-left: 16px;
+}
+
+.editor-code {
+  background-color: #f4f4f4;
+  font-family: Menlo, Consolas, Monaco, monospace;
+  display: block;
+  padding: 8px 12px;
+  line-height: 1.53;
+  font-size: 13px;
+  margin: 0;
+  margin-top: 8px;
+  margin-bottom: 8px;
+  overflow-x: auto;
+  position: relative;
+  tab-size: 2;
+  border-radius: 4px;
+}
+
+.editor-text-bold {
+  font-weight: bold;
+}
+
+.editor-text-italic {
+  font-style: italic;
+}
+
+.editor-text-underline {
+  text-decoration: underline;
+}
+
+.editor-text-strikethrough {
+  text-decoration: line-through;
+}
+
+.editor-text-code {
+  background-color: #f4f4f4;
+  padding: 1px 4px;
+  font-family: Menlo, Consolas, Monaco, monospace;
+  font-size: 90%;
+  border-radius: 3px;
+}
+
+.editor-list-ol {
+  padding: 0;
+  margin: 0;
+  margin-left: 20px;
+}
+
+.editor-list-ul {
+  padding: 0;
+  margin: 0;
+  margin-left: 20px;
+}
+
+.editor-listitem {
+  margin: 4px 0;
+}
+
+.editor-nested-listitem {
+  list-style-type: none;
+}