Draft: CODINGSTYLE.md: add formatting policy for dependencies

This MR clarifies formatting policy for dependencies in APKBUILDs. See: !28451 (comment 199801)

I believe this rule of "one item per line with closing quote on separate line" will apply to several other user defined variables as well. I'm not quite well versed with alpine packaging to know which variable definitions should follow this. So I'm open to suggestions for title of the policy (or any other changes).