fix(files): properly normalize root directory on Windows

[TheLeoP]

• I have read CONTRIBUTING.md
• I have read CODE_OF_CONDUCT.md

Fixes the issue described in #2272 based on the findings of #2272 (comment)

[TheLeoP]

Actually, after this PR, simply hovering one of C://pagefile.sys, C://swapfile.sys or C://DumpStack.log.tmp and then moving the cursor on top of any other file on the C:/ directory will result in a similar error to the one described in #2272 with the same minimal reproduction

    require("mini.files").setup {
      windows = {
        preview = true,
      },
    }
    vim.api.nvim_create_autocmd("User", {
      pattern = "MiniFilesWindowUpdate",
      callback = function()
        local s = MiniFiles.get_explorer_state()
      end,
    })

[TheLeoP]

This PR already handles the most common case of root directory normalization in Windows, but after a few iterations I ended up with

if H.is_windows then
  H.fs_normalize_path = function(path)
    path = path:gsub('\\', '/')
    local head, tail = path:match('^(%w:/+)(.*)$')
    -- preserve number of slashes if 1 or 2, normalize more than 2 slashes to 1
    if #head > 4 then head = head:gsub('/+', '/') end
    tail = tail:gsub('(.)/$', '%1')
    return head .. tail
  end
end

instead that also handles the aforementioned special paths that require specifically C:// instead of just C:/, but I'm not sure if complicating the normalization logic even more is worth it for such a niche case. It's your call @echasnovski

[nickvonkaenel]

This fix results in an error for me when using MiniFiles.reveal_cwd(). Please see the fix in my PR.