add reusable examples

[maxheld83]

No description provided.

[maxheld83]

I've built this out somewhat, still missing:

• that it also works on "normal" @examples (no reason it shouldn't just takes the last return)
• passing on @reusableExamples to @examples for all other treatment
• renaming @resuableExamples to @namedExamples which would be more appropriate.

[maxheld83]

‼️ but, more importantly, this is very likely a dead end 😔.

• you can't rely on roxygen2 comments when the package is R CMD checked, or tests are otherwise run against an installed version.
  Disallowing this / skipping (via testthat::is_checking()) would fundamentally break how tests are supposed to work.
• The path weirdness is already showing; not sure you can reliably identify the package source path from all contexts in which a test is run.
  And again, this whole thing needs the roxygen2 source to be available.
• We could write out the example calls to a separate file (just like *.Rd files get rendered), but that's just more complexity and versioning hell.
• Also, the UX is a bit awkward already.
  I think a more natural place to create the objects to be tested is inside the testthat/ folder.
• the awkwardness already shows in dealing with p