Automatically use testthat 3e

[hadley]

When creating a package and you have testthat 3.0.0 or later installed.

We may also want to provide use_testthat_3e() function.

[hadley]

(FWIW this isn't just for us, but is for everyone)

[jennybc]

I know but I was trying to find a label in addition to "feature" 😁

Seems like a good time to ask: Is this what the "code 🖥️" label is for?

[hadley]

Should just add edition argument to use_testthat and default to 3 if appropriate version of testthat found.