Use bun #3106
Description
We currently build assets on CI and include them in the git repo, so it works when running from GitHub via escript.install or MIX_ENV=prod mix phx.server, without depending on a global Node.js installation. However, with bun, it would no longer depend on anything global. Then it's less CI and less git noise.
Note: we need to make sure the assets are built as part of escript.build.