Rename branch to `main`

I'd like to propose we rename the master branch to main. The word 'master,' is charged, and its use should be discouraged. This was done for the CPython repository back in 2021 after both the community, and the Steering Council agreed this was the way to go.

GitHub has thankfully added quite considerable support for this (see the documentation), and will provide redirects/support for the old name. As such, nothing should break. (Well -- hopefully we won't break GitHub this time! ;-)

GitHub のブランチ名変更に関するドキュメントから始め、issue にリンクされている CPython の先例を確認してください。リポジトリの master ブランチが main に名前変更され、GitHub のリダイレクトとリポジトリのサポートが引き続き機能していれば完了です。