feat(platform): return PR targetBranch

lib/platform/azure/azure-helper.ts

lib/platform/bitbucket-server/utils.ts

lib/platform/bitbucket/utils.ts

lib/platform/github/index.ts

lib/platform/gitlab/index.ts

test/platform/azure/__snapshots__/azure-helper.spec.ts.snap

...latform/bitbucket-server/__snapshots__/index.spec.ts.snap

test/platform/bitbucket-server/index.spec.ts

test/platform/bitbucket/__snapshots__/index.spec.ts.snap

test/platform/bitbucket/_fixtures/responses.js

test/platform/github/__snapshots__/index.spec.ts.snap

test/platform/github/_fixtures/graphql/pullrequest-1.json

...latform/github/_fixtures/graphql/pullrequests-closed.json

test/platform/gitlab/__snapshots__/index.spec.ts.snap

test/platform/gitlab/index.spec.ts