Support system-property variable expansion in annotation attributes

[remkop]

Needed for junit-team/junit5#1498

Example use case: splitting path elements by the File.pathSeparator value. Annotations require values to be constants, so this won't compile: @Option(split = File.pathSeparator).

Anticipated usage:

@Option(names = {"-cp", "--classpath"}, 
    split = "${sys:path.separator}",
    description = "Provide additional classpath entries.")
private List<Path> additionalClasspathEntries;

[remkop]

For the junit use case we can simply specify a regex that allows both the Windows and the Unix path separator:

split = “:|;”

[remkop]

Implemented in master.