Translated the migration guide page into Japanese.

[arata-honda]

The migration guide page was great, so I quickly translated it into Japanese.
I was careful not to change the overall meaning of the translation.

state of review

• Compatibility Reference
• Source Level
• Classpath Level
• Runtime
• Metaprogramming
• Tour of the Migration Tools
• Scala 3 Migration Mode
• Migration Tutorial
• Prerequisites
• Porting an sbt Project
• Cross-Building a Macro Library
• Mixing Scala 2.13 and Scala 3 Macros
• Scala 3 Syntax Rewriting
• Incompatibility Table
• Syntactic Changes
• Dropped Features
• Contextual Abstractions
• Other Changed Features
• Type Checker
• Type Inference
• Compiler Options
• Compiler Options Lookup Table
• New Compiler Options
• Scaladoc settings compatibility between Scala2 and Scala3
• Compiler Plugins
• Kind Projector Migration
• External Resources

[eed3si9n]

コントリビューションありがとうございます。

docs.scala-lang の翻訳だとガイドと概要系はだ・である語調なので、そちらに統一した方がいいと思いますが、どうでしょうか。

[arata-honda]

ひとまずビルドは通っていないものの、2ファイルについてコメント反映させていただきました。
問題なければ残りのファイルをだ・である調に変えていきたいと思います 🙇‍♂️
@eed3si9n

[arata-honda]

@eed3si9n
一度全体的にだ・である語調に統一させていただきました。
レビューをお願いします。

[arata-honda]

@eed3si9n
コメント反映いたしました。レビューをお願いします 🙇‍♂️

[SethTisue]

where does this PR stand?

[eed3si9n]

@SethTisue I haven't taken a look at it in a while, but last I checked there it needed a lot of editing and rework (I made 76 comments in 7 pages). Anyone who "reviews" this PR would effectively have to comprehend the English original and redo some of the translation. Instead of making individual comments, I was thinking it might be better to use GitHub's review feature to stock up all comments, but it hasn't been my priority to go over the rest of 20 or so pages.

If other reviewers (@scala/docs-ja or others) want to continue reviewing, or give ok to merge as-is I won't stand in the way, but in my opinion it's not yet at the standard of other translation works that we've typically maintained.