[Dart] Support required non-scalar fields

[KirtiRamchandani]

Summary

• Treat Dart table fields marked (required) as non-null when they do not have a default value.
• Emit non-null getters, object API constructor fields, object builder fields, and raw offset builder parameters for required non-scalar fields.
• Add a Dart fixture covering a required string and a required enum vector.

Fixes #9093

Tests

• cmake -S . -B build -G Ninja -DFLATBUFFERS_BUILD_TESTS=OFF
• cmake --build build --target flatc
• ./build/flatc.exe --dart --gen-object-api -o ./dart/test ./dart/test/enums.fbs
• Regenerated enums_generated.dart matched a temp regeneration with fc.exe /b
• git diff --check

Not run: dart test because the Dart SDK is not installed in this Windows workspace I am working on atm.