Add Shop Engine Sound Preview mod with input binding, scripts, l10n and packaging

[HamiltonLockhart42]

Motivation

• Provide a Farming Simulator 25 mod that lets players preview a vehicle's engine sound in the vehicle shop before purchasing.
• Expose a dedicated input action so players can start/stop the preview from the shop HUD.
• Make the feature client-side only and safe for multiplayer/dedicated servers by avoiding server-side state changes.

Description

• Added modDesc.xml to declare the mod, register the TOGGLE_SHOP_ENGINE_PREVIEW action, and include scripts/ShopEnginePreview.lua.
• Implemented scripts/ShopEnginePreview.lua which installs hooks into ShopConfigScreen (with a runtime fallback), registers the input action, and toggles the preview vehicle's motor via startMotor/stopMotor or direct motor APIs.
• Added inputBinding.xml to bind the action to KEY_k and gamepad BUTTON_X, and created localization files l10n/texts_en.xml and l10n/texts_de.xml for action text.
• Added README.md describing features and packaging instructions and a package_mod.sh helper script to create a game-ready zip.

Testing

• No automated tests were included or run for this change.

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Codex Task