Remove implicitly appended extensions

[ash-r1]

• Sound command implicitly generates game/sounds/${filename}.mp3

  • VN.js/src/engine/commands/Sound.ts

    Line 23 in 5a8e1c2

    const src = `game/sounds/${filename}.mp3`;

• Image command implicitly generates game/images/${src}.png

  • VN.js/src/engine/commands/Image.ts

    Line 62 in 5a8e1c2

    const filepath = `game/images/${src}.png`;

But, extensions should be specified in the game script.