runnableExamples: automatically test on all backends and report which work

[timotheecour]

refs: nim-lang#15903 (comment)

[ringabout]

But runnableExamples is mainly for test. If JS backend pass but C backend doesn't pass, should CI report this?(add flags may solve it)
Another advantage is increasing the testing time.

[timotheecour]

the goal would be for docgen to report for each symbol whether it's supported on js,c,cpp,backend, without having to annotate those symbols with "proc bar is not available on VM or js backend" (or doing so at module level, which is not granular).

note that runnableExamples should primarily be used for documentation (that's guaranteed to stay in sync), and that independently of that tests should be in place to make sure tests pass on intended backends for that proc.

Another advantage is increasing the testing time.

yes; it would likely 2x or 3x the time to run the runnableExamples. but note that it hardly is a bottleneck now that runnableExamples are joined (see benchmark nim-lang#9300 (comment) or the relative times of github CI doc pipeline vs azure non-doc test pipelines)