Enable to type t.context by extending module interface

[acro5piano]

Thank you for the great tool.

Summary

This Pull Request introduces a new AvaContext interface that can be augmented by library users. By allowing developers to declare custom fields in their test context, this change provides greater flexibility and type safety when writing tests in TypeScript.

Details of Changes

• Added AvaContext Interface
  • Introduced a new AvaContext interface
  • Updated TestFn to default its generic Context to AvaContext
• Updated corresponding docs

Library users can now directly extend the AvaContext interface to add custom properties in tests, which could be impressive when using shared context:

declare module 'ava' {
  interface AvaContext {
    foo: string;
  }
}

Difference between TestFn

There is already a guide to type t.context:

const test = anyTest as TestFn<{foo: string}>;

This PR provides another method to type the context without creating a new variable, which make users ease to use context values.