Type minimap container as optional

[cmorbitzer]

Makes the minimap option container optional.

In the options passed to MinimapPlugin.create(), the container option is optional. However, the typing has it set as required. Not including it resulted in the TypeScript error TS2345: Argument of type {} is not assignable to parameter of type MinimapPluginOptions. Property container is missing in type {} but required in type WaveSurferOptions. This PR removes that error since the option is not required, and in fact, the minimap code handles the case where container is undefined.

The previous workaround was to coerce the typing: MinimapPlugin.create({} as MinimapPluginOptions)

Checklist

• This PR is covered by e2e tests
• It introduces no breaking API changes

[katspaugh]

Thanks! 👍