gatsby-plugin-graphql-codegen から gatsby-plugin-typegen へ変更

[toshifumiimanishi]

動機

gatsby-plugin-graphql-codegen is what you want. However, other maintainers and I haven't been able to keep this repo up to shape. Please have a look at @cometkim's graphql-plugin-typegen which does almost the same thing & better maintained.

gatsby-plugin-graphql-codegen の README.md に記載されている通り、今後 gatsby-plugin-graphql-codegen はメンテナンスが困難になるため同様の機能（GraphQL の型を自動で生成する）を有する gatsby-plugin-typegen に変更します。