chore: Define type for Extra(s)

[Shinigami92]

This PR introduces a type for Extra and Extras so developers can reference these types in their code

e.g.:

import { Extras } from '@sentry/types';

export class LogService {
  // ...

  public error(message: string, extras?: Extras): void {
    console.error(message, extras);
    this.message(/* ... */, extras);
  }

  // ...

  private message(/* ... */, extras: Extras = {}): void {
    // ...
    Sentry.setExtras(extras);
    // ...
  }
}

[Shinigami92]

I'm confused by the lint message

@sentry/hub: ERROR: 20:1 ordered-imports Imports from this module are not allowed in this group. The expected groups (in order) are: libraries, parent directories, current directory.

I already used vscode's Organize Imports 🤔

[kamilogorek]

Looks good, thanks!