feat: add ability to extend hermione.ctx typings

[KuznetsovRoman]

Resolves #883

Usage example

• Extend interface HermioneCtx from module "hermione":

// global.d.ts
import type { HermioneCtx } from "hermione";

declare module "hermione" {
  interface HermioneCtx {
    someVariable: string;
  }
}

• Use hermione.ctx with extended typings:

hermione.ctx.someVariable // string