vuex-class typed getters with parameters

[vuex-class](https://github.com/ktsn/vuex-class)' documentation doesn't come with clear steps on how one could call a getter with one or more arguments. 

Here's how you do it: 

```
  @workspaces.Getter
  protected workspace: WorkspaceType | null = null;
  
  public byId!: (id: WorkspaceId) => WorkspaceType;
  
  mounted(): void {
    this.workspace = this.byId({ id: this.activeWorkspaceId });
  }
```

Nick Ciolpan
18 Mar 2021