Consolidate naming for Mixin instance type

[tephenavies]

At the moment we have about 90% of Mixins using XMixin.XMixin:

namespace TableMixin {
  export interface TableMixin
    extends InstanceType<ReturnType<typeof TableMixin>> {}
  export function isMixedInto(model: any): model is TableMixin {
    return model && model.hasTableMixin;
  }
}

and 10% using XMixin.Instance:

namespace TileErrorHandlerMixin {
  export interface Instance
    extends InstanceType<ReturnType<typeof TileErrorHandlerMixin>> {}
  export function isMixedInto(model: any): model is Instance {
    return model?.hasTileErrorHandlerMixin;
  }
}

Note that removing the mixin name from the instance type name shouldn't make IDE types worse as the namespace is still displayed:

We should decide on a naming scheme, document it and apply it universally. I prefer XMixin.Instance because it's more descriptive. It's the instance type of XMixin

[nf-s]

I like XMixin.Instance over XMixin.XMixin

[nf-s]

Closed by #5661