code for dev only

[moodymudskipper]

In the same way {covr} has comments # nocov, # nocov start, # nocov end, we can have # devonly, # devonly start, # devonly end.

The code in "sysdata.rda" would skip those, but the code loaded through load_all() wouln't.

I'd use it to fire browsers in unexpected situation, and not worry about leaving them there, but can also be used to use costly assertions only in dev, or to set some dev specific defaults.