JShell API: No use of fields to return information from public types

* No use of fields to return information from public types, including enums. Using a field is over-committing to an implementation detail; use a method instead.

Enum Snippet.Status: isActive, isDefined
Enum Snippet.Kind: isPersistent
Enum SourceCodeAnalysis.Completeness: isComplete
Class SourceCodeAnalysis.CompletionInfo: all fields
Class SourceCodeAnalysis.Suggestion: all fields