Feature: Add text selection with Shift + Arrow keys

[susbakun]

Summary

Adds text selection functionality to hecto, allowing users to select text using Shift+Arrow keys while preserving syntax highlighting within the selection.

Features

• Shift+Arrow Selection: Select text using Shift + Up/Down/Left/Right/Home/End/PageUp/PageDown
• Multi-line Selection: Supports selection across multiple lines
• Syntax Highlighting Preservation: Selected text maintains its syntax colors (foreground) while showing selection background
• Selection Deletion: Pressing Delete or Backspace removes selected text
• Type-to-Replace: Typing while text is selected replaces the selection
• Dismiss Selection: Press Escape or any non-shift movement to clear selection

Keybindings

Key	Action
Shift+Left/Right	Select character
Shift+Up/Down	Select line
Shift+Home	Select to start of line
Shift+End	Select to end of line
Shift+PageUp/Down	Select page
Escape	Clear selection
Delete/Backspace	Delete selection
Any character	Replace selection

Screenshots