Add feature flagging system

[HT154]

Adds:

• New CLI flag --feature=<name>=[true|false]
• Pkl API
  • Property pkl.EvaluatorSettings#featureFlags: Mapping<KnownFeatureFlags | String, Boolean>
  • Typealias pkl.EvaluatorSettings#FeatureFlags = nothing (no feature flags yet, but this will be a string literal union)
• Java API
  • org.pkl.core.FeatureFlags - enum to document feature flags and their default values.
  • org.pkl.core.evaluatorSettings.PklEvaluatorSettings.featureFlags
  • org.pkl.core.Analyzer now requires constructor argument Map<FeatureFlag, Boolean> featureFlags
  • org.pkl.core.EvaluatorBuilder
    • addFeatureFlag, addFeatureFlags, setFeatureFlags, getFeatureFlags
    • applyFromProject(Project, Logger) (for capturing string->FeatureFlag conversion warnings)
      • The overload without the Logger argument now passes Loggers.noop() to this API
  • org.pkl.config.java.ConfigEvaluatorBuilder.applyFromProject(Project, Logger) (for capturing string->FeatureFlag conversion warnings)
    • The overload without the Logger argument now passes Loggers.noop() to this API
  • org.pkl.core.Logger (changes to assist with warnings not from Pkl code)
    • trace(String message, String frameUri) (now called by trace(String, StackFrame))
    • warn(String message, String frameUri) (now called by warn(String, StackFrame))
• Gradle tasks now accept featureFlags (as a MapProperty<String, Boolean>)
• Message passing API CreateEvaluatorRequest now accepts a map property featureFlags, which is a map of string to boolean values.

Feature flag lifecycle:

1. Feature flags are added by adding a case to org.pkl.core.FeatureFlags.
2. Flags may change default values as the language evolves.
3. Prior to removal, enum cases should be marked @Deprecated for at least one release to provide Java API users an opportunity to discontinue usage.
4. Finally, a flag is retired by removing its enum case and any code accessing its value.

Notably, because Pkl's stdlib is statically initialized with a separate VmContext, it can set its own feature flags independently of each user-initiated evaluation!

[HT154]

Whoops, we've missed updating this for a bit!