devel/nimble: Update to 0.20.0
Install bash and zsh completions.
Changelogs:
https://github.com/nim-lang/nimble/releases/tag/v0.20.0
https://github.com/nim-lang/nimble/releases/tag/v0.18.0
https://github.com/nim-lang/nimble/releases/tag/v0.16.4
PR: 288080
Approved by: ports@nicandneal.net (maintainer)