The default SmartFormatter used by the static Smart formatting methods.
If not set, a formatter from Smart.CreateDefaultSmartFormat is used.
Using the ThreadStaticAttribute, Smart.Default returns isolated instances of the SmartFormatter in each thread.
As Smart.Default is thread-static, customizations must be applied on each thread.
Note that the internal object pools are shared across all threads and are not thread safe,
so formatting from multiple threads concurrently is not supported.