Support extended package validators

[pi0]

trafficstars

Describe the feature

Unbuild has built-in basic validation for package exports. Which we could improve to cover more cases. Two open issues:

• https://github.com/unjs/unbuild/issues/4
• https://github.com/unjs/unbuild/issues/22

However there are also nice utils specific to this purpose we can have opt-in support for them (via config + optional deps)

Some options:

• arethetypeswrong
• publint
• sherif

[!NOTE] Please do not raise a PR yet, I'm evaluating options and ideas, which are more than welcome on this topic!

Additional information

• [ ] Would you be willing to help implement this feature?