Disable following links (#884)

Following links resulted on my machine (running NixOS and using devenv)
in precompilation attempting to format read-only .jl files (because
`.devenv/profile` links to readonly `/nix/store/…` directories). Since I
don't see a case where you may want JuliaFormatter to jump to far-away
directory by following links (looks rather dangerous to me, actually?),
I simply disabled following links.

Note this could be improved by checking whether the link points to a
file within the currently-to-be-formatted directory.

src/JuliaFormatter.jl

@@ -427,7 +427,7 @@ function format(path::AbstractString, options::Configuration)
         formatted = Threads.Atomic{Bool}(true)
         Threads.@threads for subpath in readdir(path)
             subpath = joinpath(path, subpath)
-            if !ispath(subpath)
+            if !ispath(subpath) || islink(subpath)
                 continue
             end
             is_formatted = format(subpath, options)