`ISO8601DateFormatter.Options` should have immutable static properties

[fumoboy007]

ISO8601DateFormatter.Options uses var for its static properties. The properties should instead be declared using let to prevent accidental mutation and to let the compiler know that these properties are concurrency-safe.

Regarding the latter point, the following is an example error caused by this issue:

error: reference to static property 'withInternetDateTime' is not concurrency-safe because it involves shared mutable state