[IMP] component: display nice error for wrong child component

If you declare a child component which is not actually a Component,
the error message is not very friendly and not very helpfull to find
what happens and which child component is wrong.

```
const ChildComponent = "not a component constructor";

class MyComponent extends Component {
    static components = { ChildComponent };
}
```

This commit improves the type declaration for those working with Typescript
and adds a runtime check for javascript codebases

src/component/component.ts

@@ -10,6 +10,7 @@ interface StaticComponentProperties {
   template: string;
   defaultProps?: any;
   props?: any;
+  components?: { [componentName: string]: ComponentConstructor };
 }
 
 export type ComponentConstructor<P extends Props = any, E = any> = (new (

src/component/component_node.ts

@@ -107,6 +107,8 @@ export function component<P extends object>(
       C = parent.constructor.components[name as any];
       if (!C) {
         throw new Error(`Cannot find the definition of component "${name}"`);
+      } else if (!(C.prototype instanceof Component)) {
+        throw new Error(`"${name}" is not a Component. It must inherit from the Component class`);
       }
     }
     node = new ComponentNode(C, props, ctx.app, ctx, key);