Add command to update SwiftFormat reference to latest version

[calda]

This PR adds a bundle exec rake update:swiftformat command that updates the SwiftFormat dependency Package.swift to reference the most recent version published in the https://github.com/calda/SwiftFormat repo.

This combines nicely with nicklockwood/SwiftFormat#1721, which automates the creation of the artifact bundle.

If you run this right now, the command applies this change to the Package.swift file:

diff --git a/Package.swift b/Package.swift
index 579c10c..7b2daee 100644
--- a/Package.swift
+++ b/Package.swift
@@ -42,8 +42,8 @@ let package = Package(
 
     .binaryTarget(
       name: "SwiftFormat",
-      url: "https://github.com/calda/SwiftFormat/releases/download/0.54-beta-7/SwiftFormat.artifactbundle.zip",
-      checksum: "0cf117050e7838f545009bfe4a75dbda98cff737cb847a7d065a89683e9e890a"),
+      url: "https://github.com/calda/SwiftFormat/releases/download/0.54-test-upload-release-artifact-3/SwiftFormat.artifactbundle.zip",
+      checksum: "45eb713635edde284f06c2aa0c556f25336cbb382ae0e2226553abf468092e63"),
 
     .binaryTarget(
       name: "SwiftLintBinary",