feat: add H3EventContext for augmentation

[danielroe]

Simply exposes an EventContext interface for manual typing:

declare module 'h3' {
  interface EventContext {
    myProperty?: boolean
  }
}
export {}

resolves #122

[pi0]

(I'm not sure but kinda feeling using H3EventContext would be less confusing in the future about naming interface with H3 prefix. Currently some exports are prefixed and some not)

[wobsoriano]

Can we also add augmentation for the request object? This is for using connect/express middlewares.

An example with express-session:

// ~/server/middleware/session.ts
import session from 'express-session'
import type { Session } from 'express-session'

export default session(options)

declare module 'h3' {
  interface EventRequest {
    session: Session
    sessionId: string
  }
}

// ~/server/api/user.ts
export default defineEventHandler((event) => {
  event.req.session // typed properly!
})

Current workaround:

declare module 'h3' {
  interface CompatibilityEvent {
    event: CompatibilityEvent
    req: IncomingMessage & {
      session: Session
      sessionId: string
    }
    res: ServerResponse
    context: Record<string, any>
  }
}