This commit allows users of this code to add extra directories to be ignored
during project loading, which can be extremely useful while also using toolsets
like Nix (which may symlink other F# projects in to the repo, thus confounding
project loading).

This commit implements an approach proposed in emacs-lsp#3906. Specifically, it provides
new control variables to make it possible to use C# and F# language servers
_either_ as global _or_ local dotnet tools. So far, I've done this by providing
boolean variables to control command construction, but I'm not attached to this
approach.

Closes emacs-lsp#3906.