Define a type that represents a plugin class

[RolandJansen]

At several locations in the code there is this typedef for plugin classes:

new (itemId: string, ac: AudioContext) => IPlugin

this could be refactored as a type or interface for better readability.

[RolandJansen]

Added this as an interface because then we can define static members like metaData:

export interface IPluginConstructor {
    readonly metaData: IPluginMetaData;
    new (itemId: string, ac: AudioContext): IPlugin;
}