[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/store_hooks.test.ts

@@ -1,5 +1,5 @@
 import { Component, Env } from "../src/component/component";
-import { Store, useStore, useDispatch, useGetters } from "../src/store";
+import { Store, useStore, useDispatch, useGetters, EnvWithStore } from "../src/store";
 import { useState } from "../src/hooks";
 import { xml } from "../src/tags";
 import { shallowEqual } from "../src/utils";
@@ -885,7 +885,7 @@ describe("connecting a component to store", () => {
       actions
     });
 
-    class TodoItem extends Component {
+    class TodoItem extends Component<{}, EnvWithStore> {
       static template = xml`
             <div class="todo">
                 <t t-esc="state.todo.title"/>
@@ -958,7 +958,7 @@ describe("connecting a component to store", () => {
       actions
     });
 
-    class TodoItem extends Component {
+    class TodoItem extends Component<{}, EnvWithStore> {
       static template = xml`
             <div class="todo">
                 <t t-esc="state.todo.title"/>