feat: Add TextField widget for text input

[Moon-DaeSeung]

Summary

Add a TextField widget that allows users to input and edit text, enabling form-based interactions within Flitter applications.

Motivation

Text input is one of the most fundamental UI primitives. Without a TextField widget, Flitter cannot support forms, search bars, chat inputs, or any interactive text entry. This is essential for building complete applications.

Proposed Widgets

• TextField — A basic text input widget. Equivalent to Flutter's TextField.
• TextEditingController — A controller for the text being edited, providing programmatic access to the text value.
• InputDecoration — Styling configuration for the text field (hint text, borders, labels, icons).

Key Considerations

• Cursor rendering and positioning
• Text selection (click, drag, shift+arrow keys)
• Keyboard input handling (including IME/composition for CJK languages)
• Copy/paste support
• Focus management (FocusNode integration)
• Styling: border, hint text, label, prefix/suffix icons
• Multiline support (optional)
• Integration with both SVG and Canvas renderers
• Accessibility considerations

References

• Flutter TextField
• Flutter TextEditingController