feat: Add LocalsExt interface

[NathanHuisman]

PR Type

• Feature

What is the current behavior?

I found myself struggling to extend FetchEvent.locals globally, because typescript does not allow declaration merging of non-method members of interfaces (see https://www.typescriptlang.org/docs/handbook/declaration-merging.html#merging-interfaces)

What is the new behavior?

We can improve this by adding a LocalsExt interface that can be merged in your own app.d.ts like this:

import type { LocalsExt } from "solid-start/server";

declare module "solid-start/server" {
  interface LocalsExt {
    auth?: AuthUser;
  }
}

Other information

Issue #848

I saw Lucia doing something similar with a namespace

[ryansolid]

Ok thank you.