Release 1.4

• Adopt the Swift 6 language mode. All public value types are now Sendable (CLControl, Directive, Parameters, Number, Currency, Context, ...), as are the error types and CLFormatConfig. A parsed CLControl can be formatted concurrently from several threads.
• DirectiveSpecifier now refines Sendable. Custom directive specifiers have to be Sendable, i.e. they must not have mutable state.
• Make CLFormatConfig.default thread-safe. Changing it while formatting on other threads used to be a data race.
• Fix displayWidth being ignored inside iterations (~{, ~:{, ~@{) and indirections (~?)
• Fix NSDecimalNumber arguments being converted to Double, which lost precision
• Report the parameters of malformed closing directives (e.g. ~1)) in the error message
• Add DocC documentation, including a complete reference of all directives and articles on getting started and on adding custom directives, and installation instructions to the README
• Document directive ~ followed by a newline (line continuation) in the directive reference, and correct a few errors in it (the results of the Pairs examples of ~{, and the conversion method listed for CLFormatConvertible in ~A)
• Extend the test suite considerably, in particular for errors, currencies, and less common directives