Constructs a DateTimeFormatter using all the appended elements.
This is the main method used by applications at the end of the build process to create a usable formatter.
Subsequent changes to this builder do not affect the returned formatter.
The returned formatter may not support both printing and parsing. The methods {@link DateTimeFormatter#isPrinter()} and{@link DateTimeFormatter#isParser()} will help you determine the stateof the formatter.
@throws UnsupportedOperationException if neither printing nor parsing is supported