add variable to ignore specific projects

[dradetsky]

I want to add a git repo to my home directory in order to track dotfiles (I'm aware there are other approaches that don't involve creating ~/.git/, but for reasons beyond the scope of this PR, I want to do it this way). When I do this, in doom-emacs doom doctor complains

    ! Your $HOME is recognized as a project root
      Emacs will assume $HOME is the root of any project living under $HOME. If this
      isn't desired, you will need to remove ".git" from
      `projectile-project-root-files-bottom-up' (a variable), e.g.
      
        (after! projectile (setq projectile-project-root-files-bottom-up (remove ".git"
          projectile-project-root-files-bottom-up)))

Maybe I'm misunderstanding something, but it seems like the resolution here isn't something I want to do. I still want projectile to recognize dirs with .git as project roots, I just want it to not consider $HOME to be the universal project root, as threatened by doom doctor. Therefore, I need it to say that one or more named directories are not project roots, regardless of what the root-determination machinery would say otherwise.

So I add a variable to hold a list of such directories, and consult it inside projectile-locate-dominating-file.

NOTE: the PR is intentionally missing some required cleanup (like comments) in case there is something fundamentally wrong with my whole approach (I suck at elisp). That's why it's marked as a draft. AFAIK, the code itself does what it should, but I could be wrong.

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