Bundler environment variable inlining should be configurable

[mangs]

What is the problem this feature would solve?

In testing our Bun-based lambda while prepping its rollout to production, I realized that despite the documentation saying that environment variable inlining occurs when building, the bundler actually doesn't. This resulted in my PR to remove from the documentation any mention of environment variable inlining until the feature is added back somehow. Being able to choose whether or not this is done would be really helpful and would be a nice way to accomplish the feature without forcing anyone into behavior they don't want.

For frontend apps, having environment variables inlined is helpful. However, in the case of server-side apps on Node.js or Bun, being able to read environment variables at runtime is helpful. This is especially so on serverless platforms with shared execution environments where secret values are best accessed via environment variables.

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

A configuration flag for the bundler to enable/disable environment variable inlining. For example:

bun build --inline-env-vars[=y,n]

Perhaps target: 'browser' could inline by default and target: 'bun' and target: 'node' could disable inlining, although inconsistent/unexpected behavior between targets will probably confuse more than help; just mentioning this for completeness in case I'm not thinking of a better option.

What alternatives have you considered?

Manual value replacement with a text replacement engine like sed