[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.

tests/component/component.test.ts

@@ -3484,11 +3484,14 @@ describe("environment and plugins", () => {
   });
 
   test("can define specific env for root components", async () => {
-    class App1 extends Component {
+    interface AppEnv extends Env {
+      test: number;
+    }
+    class App1 extends Component<{}, AppEnv> {
       static template = xml`<span></span>`;
     }
     App1.env = { test: 1 };
-    class App2 extends Component {
+    class App2 extends Component<{}, AppEnv> {
       static template = xml`<span></span>`;
     }
     App2.env = { test: 2 };