fix(gatsby): add this typings to actions

[alvis]

Description

Commonly, we destruct the action field as well as other helper functions from the API argument like this:

const {
  actions: { createNode, deleteNode },
  createContentDigest,
  createNodeId,
  reporter,
} = args;

These functions are bounded and it's 100% safe to invoke these functions since they do not access this.
However, the current type file doesn't explicitly state that and hence would trigger a warning from some linting tools such as @typescript-eslint/unbound-method

This PR declares it explicitly and therefore avoid unnecessary warnings
See https://github.com/typescript-eslint/typescript-eslint/blob/v4.28.0/packages/eslint-plugin/docs/rules/unbound-method.md

[alvis]

ping @wardpeet @LekoArts @KyleAMathews @pieh 🥺

[wardpeet]

Thanks, looks great! Sorry for the wait!