Add keybindings for move/select by word

[crijke]

This PR restores keybindings to move the cursor by one word and to select text by words, mapping them to the MacOS equivalents:

• option + cursor right: move cursor one word to the right
• option + cursor left: move cursor one word to the left
• shift + option + cursor right: expand text selection one word to the right
• shift + option + cursor left: expand text selection one word to the left

[Copilot]

Pull Request Overview

This PR adds keyboard shortcuts for word-based cursor movement and text selection, mapping MacOS-style Option+Arrow key combinations to their Linux equivalents using Ctrl+Arrow keys.

• Removes TODO comments for word navigation functionality
• Adds new keymap section with Alt+Arrow remappings to Ctrl+Arrow
• Includes both movement-only and Shift+modifier combinations for text selection

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