Use nimsuggest v3 when nim version is >=1.6

[vanyle]

See title. Version 3 of nimsuggest is faster (allegedly)

nimsuggest is very poorly documented. I might need to write a documentation somewhere. These PRs contain some docs: https://github.com/nim-lang/Nim/pull/19826 https://github.com/nim-lang/Nim/pull/19892