Update to SwiftFormat 0.56-beta-4 (airbnb#296)

Add rule to use doc comments before declarations (airbnb#262)

Add rules for how to wrap attributes on property declarations (airbnb…

…#254)

Add rule to prefer for loops over functional `forEach(…)` method (air…

…bnb#234)

Test multiple Xcode versions in CI (airbnb#224)

Update rule for long function declarations to specify effects (`async…

…`, `throws`) should be written on the line after the closing paren (airbnb#205)

Add rule to prefer opaque generic parameter syntax (airbnb#193)

Infer Swift version from Package.swift swift-tools-version (airbnb#180)

Add info about Swift Package Manager command plugin to README.md (air…

…bnb#176)