Allow shared code based on {staticimports}

[gadenbuie]

We occasionally need utility functions and shared code between apps, but we don't want to introduce a dependency on shinycoreci in the test code. Ideally, testing apps should be completely isolated.

We could build on staticimports to store shared code in {shinycoreci}/inst/staticexports that is then "vendored" into the test app folders.

We'd also need a shinycoreci function that walks the test apps and can update any out-of-date imports.