refacto: create actions folder and move some code to it

- move trash.lua into actions
- move system-open from root file to actions
- move copypasta code from fs to actions
- add file_exists in utils
- add rename_loaded_buffers in utils (might need to move that in the
  future).

lua/nvim-tree/actions/system-open.lua

@@ -0,0 +1,68 @@
+local uv = vim.loop
+
+local M = {
+  config = {
+    is_windows = vim.fn.has('win32') == 1 or vim.fn.has('win32unix') == 1,
+    is_macos   = vim.fn.has('mac') == 1 or vim.fn.has('macunix') == 1,
+    is_unix    = vim.fn.has('unix') == 1,
+  }
+}
+
+function M.fn(node)
+  if not M.config.system_open.cmd then
+    require'nvim-tree.utils'.warn("Cannot open file with system application. Unrecognized platform.")
+    return
+  end
+
+  local process = {
+    cmd = M.config.system_open.cmd,
+    args = M.config.system_open.args,
+    errors = '\n',
+    stderr = uv.new_pipe(false)
+  }
+  table.insert(process.args, node.link_to or node.absolute_path)
+  process.handle, process.pid = uv.spawn(process.cmd,
+    { args = process.args, stdio = { nil, nil, process.stderr }, detached = true },
+    function(code)
+      process.stderr:read_stop()
+      process.stderr:close()
+      process.handle:close()
+      if code ~= 0 then
+        process.errors = process.errors .. string.format('NvimTree system_open: return code %d.', code)
+        error(process.errors)
+      end
+    end
+  )
+  table.remove(process.args)
+  if not process.handle then
+    error("\n" .. process.pid .. "\nNvimTree system_open: failed to spawn process using '" .. process.cmd .. "'.")
+    return
+  end
+  uv.read_start(process.stderr,
+    function(err, data)
+      if err then return end
+      if data then process.errors = process.errors .. data end
+    end
+  )
+  uv.unref(process.handle)
+
+end
+
+function M.setup(opts)
+  M.config.system_open = opts or {}
+
+  if not M.config.system_open.cmd then
+    if M.config.is_windows then
+      M.config.system_open = {
+        cmd = "cmd",
+        args = {'/c', 'start', '""'}
+      }
+    elseif M.config.is_macos then
+      M.config.system_open.cmd = 'open'
+    elseif M.config.is_unix then
+      M.config.system_open.cmd = 'xdg-open'
+    end
+  end
+end
+
+return M