Debug log that development build is being used

[timfish]

Description

Where we have separate debug/production builds that are selected via package.json exports, we should log that the development output is being used!

Without this, there is no way to know which build output a user is running which may be important when users report an issue.

For example, if someone reports that Spotlight isn't working we have no easy way to determine whether their bundler is actually picking up the development build!

Cc @BYK

[linear[bot]]

JS-1258