feat: Refactor italic node to support nested inline elements

[singhanubhavme]

This PR introduces a significant improvement to the italic node structure by refactoring it to use a child-based content representation instead of a simple string. This change enables better support for nested inline elements within italic text.

Key changes:

• Modified ast.Italic struct to use Children []Node instead of Content string
• Updated the italic parser to parse nested inline elements (links, text) within italic content
• Updated HTML and String renderers to handle the new node structure
• Fixed related test cases in ordered list items

Benefits:

• Better support for complex markdown structures
• More consistent with the AST design pattern used elsewhere in the codebase
• Improved extensibility for future inline element additions
• Maintains backward compatibility with existing markdown parsing

The changes have been tested thoroughly with various markdown inputs, including:

• Basic italic text with asterisks and underscores
• Nested inline elements (links within italic text)
• Mixed content with spaces and special characters

Other Changes:

• Bold and Italic via underscores supported, empty blockquotes allowed.

Note:
This change also required a corresponding update in the main repository [https://github.com/usememos/memos], where this package is being used. A PR has been created there as well to ensure compatibility with the new Italic node structure.

[boojack]

LGTM