Expose RequestError

[georeith]

This PR exposes RequestError which is the error class used by any of the octokit request functions:

try {
   await octokit.rest.git.getRef({
      owner,
      repo,
      ref,
    });
  } catch (err: unknown) {
    if (err instanceof RequestError && err.status === 404) {
      // ...
    }
  }

It is important that this is exposed by this library because it originates from the @octokit/request-error package, however if the user installs the wrong version of @octokit/request-error that does not match the one imported by this library via @octokit/core then you are unable to use instanceof checks to narrow this error.

As an end user of this library it is not easy to keep these versions in lockstep as you do not import @octokit/core directly and this library does not version in lockstep with @octokit/core so it is greatly simplified and less error prone if this library just exports the specific class it already uses.