Handle multiple test frameworks when exporting to `SBT`/`Mill`

[Gedochao]

Is your feature request related to a problem? Please describe. Currently, the export sub-command only exports a single test framework configuration, regardless how many are actually detectable.

Describe the solution you'd like I'd like to export whatever information on test framework Scala CLI can extract, or at least warn about what needs to be adjusted manually.

Additional context

• https://github.com/VirtusLab/scala-cli/pull/3621