Add Shop Engine Sound Preview mod with input binding and packaging script

[HamiltonLockhart42]

Motivation

• Provide an in-shop engine sound preview so players can hear a vehicle's engine before purchase.
• Allow quick start/stop of the preview via a configurable input action and ensure the preview stops when switching vehicles or closing the shop.
• Keep the feature client-side and multiplayer-safe by avoiding server-side state changes.

Description

• Added modDesc.xml and scripts/ShopEnginePreview.lua which implement the ShopEnginePreview class that hooks ShopConfigScreen to register an input action and toggle preview engine start/stop.
• Implemented preview logic to detect the preview vehicle, start/stop the motor via startMotor/stopMotor or fallback to motor getters/setters, and automatically stop preview on vehicle switch or shop close.
• Added default input bindings in inputBinding.xml (keyboard K, gamepad X) and localization files l10n/texts_en.xml and l10n/texts_de.xml for action text.
• Included package_mod.sh to produce a game-ready zip and updated README.md with features, packaging instructions, and mod file structure.

Testing

• No automated tests were executed for this change.

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Codex Task