Switch to using `projen` for each package tested

[moltar]

Using projen would open up the possibility of having different configs for each package that is being tested.

This would be necessary for #898, but we have also ran into this with spectypes (#856).

• node_modules Isolation #1219

[johannes-lindgren]

When trying to add a new benchmark, I noticed that I couldn't use package.json "exports". It was difficult to fix because a change in project configuration would also affect other benchmarks. Maybe this issue would help solving that.

[moltar]

@johannes-lindgren What was the use case for exports?