esbuild / rollup / webpack all use yarn instead of npm as the default package manager:

• https://github.com/rails/jsbundling-rails/blob/main/lib/install/esbuild/install.rb#L5
• https://github.com/rails/jsbundling-rails/blob/main/lib/install/rollup/install.rb#L6
• https://github.com/rails/jsbundling-rails/blob/main/lib/install/webpack/install.rb#L6

Given some of the issue surrounding yarn v1 / v2 (npm install -g yarn still installs v1.22.22), propose swapping the default package manger to just use npm directly since:

1. It removes a dependency on yarn.
2. Avoids the yarn classic vs yarn modern issues.