Add Shop Engine Sound Preview mod with input action, script, localization, and packaging

[HamiltonLockhart42]

Motivation

• Provide a client-side feature that lets players start/stop the preview vehicle engine sound from the vehicle shop to hear it before purchasing.
• Integrate a configurable input action and localizations so the feature works out-of-the-box and is translatable.

Description

• Add modDesc.xml to register the TOGGLE_SHOP_ENGINE_PREVIEW action, include scripts/ShopEnginePreview.lua, and declare inputBinding.xml and localization files.
• Implement scripts/ShopEnginePreview.lua which hooks ShopConfigScreen (onOpen, onClose, update), registers the input action, and toggles the preview vehicle motor via vehicle:startMotor()/stopMotor() or the vehicle motor API while avoiding dedicated servers.
• Add inputBinding.xml with default bindings (KEY_k and BUTTON_X) and localization files l10n/texts_en.xml and l10n/texts_de.xml for the action labels.
• Add package_mod.sh to create a game-ready zip and update README.md with usage, controls, file structure, and packaging instructions.

Testing

• No automated tests were run.

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Codex Task