Fix enum value features and STRICT visibility edition 2024

[emcfarlane]

This fixes three edition 2024 behaviors.

Enum value features were ignored. Feature lookup on builtins.EnumFeatures instead of builtins.EnumValueFeatures, so features set on an enum value were never applied or validated against the edition.

STRICT nested visibility. protoc fixed its namespaced-enum check upstream (protocolbuffers/protobuf@1229d4a, protocolbuffers/protobuf@18561e4), so no longer need the bugged warning workaround. Under STRICT, nested messages can never be exported. A nested enum can only be exported if it is inside a top-level local message with a single reserved 1 to max;.

Source spans for export/local types. protoc consumes the visibility modifier before recording a message or enum's location, so the span starts at message/enum. Comments still attach from the first token.