feat: add tasks for ARC-like contests

[KATO-Hiro]

close #4140

Summary by CodeRabbit

• 新機能
  • DDCC 2020予選、日経プログラミングコンテスト2019-2予選、Diverta 2019-2のA〜Fの問題情報を追加しました。
  • これらのコンテストがARC系として扱われるようになりました。

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📥 Commits

Reviewing files that changed from the base of the PR and between bb0b2a4 and cb44b3d.

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📒 Files selected for processing (3)

• prisma/tasks.ts
• src/lib/contests/fixtures/contest_type.ts
• src/lib/contests/utils/prefixes.ts

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[KATO-Hiro]

LGTM