[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

tests/components/__snapshots__/error_handling.test.ts.snap

@@ -1,5 +1,16 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`basics display a nice error if a component is not a component 1`] = `
+"function anonymous(bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, component, comment } = bdom;
+  
+  return function template(ctx, node, key = \\"\\") {
+    return component(\`SomeComponent\`, {}, key + \`__1\`, node, ctx);
+  }
+}"
+`;
+
 exports[`basics display a nice error if it cannot find component (in dev mode) 1`] = `
 "function anonymous(bdom, helpers
 ) {