title: Extending charjs interface with missing types
body: <p>Im am trying to set scaleDefaults for chartjs. I am using the typings from  <code>@types/chart.js</code> v2.7.22.</p>
  
  <pre><code>Chart.scaleService.updateScaleDefaults('logarithmic', {
      ticks: {
          callback: function (tick: number, index: number, ticks: number[]) {
              // CODE
          }
      }
  </code></pre>
  
  <p>But this however gives an issue: <code>Property 'scaleService' does not exist on type 'typeof Char'</code>. If i cast this to <code>any</code> however it works fine.</p>
  
  <p>But i dont wan to cast this to any so i'd wanted to extend the chartjs interface resulting in:</p>
  
  <pre><code>interface ChartExtended extends Chart { }
  
  (Chart as ChartExtended).scaleService.updateScaleDefaults('logarithmic', {
      ticks: {
          callback: function (tick: number, index: number, ticks: number[]) {
              // CODE
          }
      }
  </code></pre>
  
  <p>But again i am confronted with type errors: </p>
  
  <pre><code>Type 'typeof Chart' cannot be converted to type 'ChartExtended'.
  Property 'config' is misisng in type 'typeof Chart'
  </code></pre>
  
  <p>When i check the index.d.ts I notice that there is a seperate class def and a namespace def with the same name:</p>
  
  <pre><code>declare class Chart {
      static readonly Chart: typeof Chart;
      // Stuff
  }
  
  declare namespace Chart {
       // Stuff
  }
  
  export = Chart;
  export as namespace Chart;
  </code></pre>
  
  <p>I want to create an interface that properly extends chart so that i can define scaleService.</p>
  
  <p>I'd expect to defined the interface like:</p>
  
  <pre><code>interface ChartExtended extends Chart {
      scaleService?: ScaleService
  }
  </code></pre>