use package extensions for staticarrays

[aplavin]

Following https://pkgdocs.julialang.org/dev/creating-packages/#Conditional-loading-of-code-in-packages-(Extensions).
Uses Requires as before on 1.8-, and uses the new extensions mechanism instead when it's supported.

[jw3126]

Awesome! I never tried the new conditional extensions before.

[aplavin]

I asked on slack if the implementation is correct, and @KristofferC says it's fine.

AFAIU, Registrator doesn't support weak deps yet, so I won't merge/register this for now.
Maybe, I'll add some more straightforward package integrations in the meantime... No Requires, only extensions - so they would only work on 1.9+.

[aplavin]

Added 1.9 to CI to ensure extensions work - regular nightly (1.10) has an inference regression failing another test.

Looking at General, registrator now supports weak deps, so my plan is to pull the trigger on this soon... No hurry ofc, not even 1.9-rc is out yet.