Add npm_config_user_agent when running bun create

[C0RR1T]

What is the problem this feature would solve?

Other package managers (like pnpm, yarn and the like) inject npm_config_user_agent into process.env when running scripts. this makes it really easy for packages to differentiate which package manager was used to execute the script. Bun also seems to do this in some cases, but not in bun create (didn't test any command besides bun run, I've heard that it works with bunx aswell.

What is the feature you are proposing to solve the problem?

Add the bun user agent via the process.env.npm_config_user_agent variable when running bun create

What alternatives have you considered?

No response