Add SelectableText widget

[flakerimi]

Summary

• Adds SelectableText widget to ShaftKit — a read-only text widget that supports user selection and copy
• Built on top of EditableText with readOnly: true, hidden cursor, and text selection gesture support
• Includes a Playground demo page with examples: basic usage, styled text, multiline, custom selection color, and comparison with regular Text

Details

SelectableText mirrors Flutter's SelectableText — it wraps EditableText configured as read-only with selection enabled. Supports:

• style, textAlign, textDirection, maxLines
• selectionColor for custom highlight color
• onSelectionChanged callback
• focusNode for focus management
• Inherits from DefaultTextStyle

Test plan

• Build compiles without errors
• Playground page renders and text is selectable via click-and-drag
• Styled and multiline variants work correctly
• Custom selection color applies
• Regular Text remains non-selectable for comparison