feat(swift): Support for Package.swift files (#3911)

lib/config/definitions.js

lib/datasource/git-tags/index.js

lib/manager/index.js

lib/manager/swift/extract.js

lib/manager/swift/index.js

lib/manager/swift/update.js

lib/versioning/swift/index.js

lib/versioning/swift/range.js

renovate-schema.json

test/config/__snapshots__/validation.spec.js.snap

test/datasource/git-tags.spec.js

test/manager/swift/__snapshots__/index.spec.js.snap

test/manager/swift/_fixtures/SamplePackage.swift

test/manager/swift/index.spec.js

test/versioning/swift.spec.js

...rkers/repository/extract/__snapshots__/index.spec.js.snap

website/docs/configuration-options.md