[IMP] types: do not make Env an indexed type

Before this commit, Env was an indexed type, this means that one could
write env.anything, and it would accept it as a valid type. This is
actually quite dangerous, because we lose the typing advantages for all
keys that are properly defined.

For example, if a component is defined as:

class MyComponent extends Component<Props> {
 ...
}

Then Typescript will let it use anything from the environment, even if
it is wrong.  So, most properly typed Typescript applications should use
instead a sub environment:

interface MyAppEnv extends Env {
  someKey: someValue
}

Then, the component should be defined this way:

class MyComponent extends Component<Props, MyAppEnv> {
 ...
}

Before this commit, any typos in the environment accesses would not be
noticed by typescript.

src/component/component.ts

@@ -34,7 +34,6 @@ import { activateSheet } from "./styles";
  */
 export interface Env {
   qweb: QWeb;
-  [key: string]: any;
 }
 
 export type MountPosition = "first-child" | "last-child" | "self";