Feature: Allow for multiple plugins of the same type

[Zyie]

plugins: {
        texture: texturePacker({
            texturePacker: {
                exporter: 'Pixi',
            }
        }),
        texture2: texturePacker({
            texturePacker: {
                exporter: 'Css',
            }
        }),
        manifest: pixiManifest({}),
    },

This should produce both css and json outputs, both being saved to the manifest

Currently this causes a key clash in SaveableAssetCache