Add repository preferences to the context

[dsame]

To solve this issue actions/stale#881 it is necessary to detect is the current repository forked from the another.

This PR introduces new getter to github's Context called "repositoryPreferences" with the only property "isFork".

Alternatives:

1. to do not introduce this property at all (cancel this PR), just access the payload directly from actions/stale repo. PROS: simplicity, consistency with GitHub REST API, CONS: actions/stale never accesses payload property of the context.
2. to add isFork to repo getter (not introduce repositoryPreferences). PROS: seems to be obvious, CONS: I assume semantic of the repo is not "repository" but rather "id of the repository". Particularly repo may exist before the payload initialised.
3. to rename repositoryPreferences to repository and isFork to fork. PROS: to be in consistency with GitHub REST API, CONS: it is not clear what would be difference between repo and repository; from the fork name it is not obvious the property is boolean.

[dsame]

It was decided to use 1st alternative