Clarify relation to `@testitem` from TestItems.jl

[BeastyBlacksmith]

Its not clear to me, whether these are related or not and whether the VSCode integration works the same.

[nickrobinson251]

The aim is for these to be compatible with VSCode integration (i.e. have the same interface)

One day we hope they'll be combined into a single codebase, so there's just one implementation (#18)