[Plugin Request] Testing plugins

[Wollantine]

Being able to work convention-based, without bundler configuration, is awesome, and I was enjoying @pika/pack a lot... Until I had to install all the environment anyway to be able to run the tests. Babel, its adapters, everything, just for running the tests.

As I want to believe that every single worthy library is nowadays being produced with unit tests, I think that this renders pack pretty useless, so I wanted to suggest the idea of making plugins to support some kind of testing.

The tests could run during pack build and break the process the same way tsc does when it doesn't compile when using @pika/plugin-ts-standard-pkg.

Some plugin suggestions would be:

• plugin-mocha
• plugin-ts-mocha
• plugin-jest
• plugin-ts-jest

[FredKSchott]

This is a really interesting idea. It doesn't really map to the current implementation (all build plugins are meant to modify the pkg/ directory for publishing) but it's not a huge stretch to imagine a plugin that read/loaded code from the pkg/ directory, "Did Something" (ex: ran tests), and then passed/failed as needed.

Relevant conversation going on here as well: https://github.com/pikapkg/pack/issues/29#issuecomment-493786715