Feature: Add support for repository variables

[primetheus]

This PR adds support for repository-level variables that can be used outside of a specific deployment environment.

cc: @decyjphr

[decyjphr]

The code was failing the tests, perhaps the find method could be rewritten as

Suggested change

	const result = async () => {
	async find () {
	const res = await this.github.request('GET /repos/:org/:repo/actions/variables', {
	org: this.repo.owner,
	repo: this.repo.repo
	})
	const properties = res.data.map(d => { return { name: d.property_name, value: d.value } })
	return properties
	}

[PendaGTP]

@primetheus could you also update the related docs? Like provide examples to make the distinction between environment variables and GH Actions variables more clear (#820)?