use symlinks instead of shims where symlinks are supported
on OS that always support symlinks (most OS's including windows 10), choosenim should use symlinks instead of shims.
Shims cause performance overhead (if your nim command invocation is a simple one, eg `nim dump` or `nim --version` etc), and more importantly can cause issues, eg https://github.com/dom96/choosenim/issues/126 in the past but there are others.

another example is it's not easy from just `which nim` to tell which path is the underlying nim. I intend to add `path/to/nim` in `nim dump` but that's just a workaround.

The only question is what to do about windows, whether to auto-detect whether to use shims vs symlinks (depending on windows version), or (simpler IMO), let user choose via `choosenim --usesymlink:on/off [args...]`

## notes
* recently discussed on IRC https://gitter.im/nim-lang/Nim?at=5e89179230ec2e5a259b2b25 https://irclogs.nim-lang.org/04-04-2020.html#23:25:07
* windows10 supports non admin symlinks since 2016 https://www.wintellect.com/non-admin-users-can-now-create-symlinks-windows-10/