fix: replace vim.* "requires" with explicit calls to vim functions (#1701)

lua/nvim-tree/actions/init.lua

@@ -1,7 +1,5 @@
 -- @deprecated: new implementation in nvim-tree.keymap. Please do not edit this file.
 
-local a = vim.api
-
 local log = require "nvim-tree.log"
 local view = require "nvim-tree.view"
 local notify = require "nvim-tree.notify"
@@ -350,7 +348,7 @@ end
 
 local function cleanup_existing_mappings()
   local bufnr = view.get_bufnr()
-  if bufnr == nil or not a.nvim_buf_is_valid(bufnr) then
+  if bufnr == nil or not vim.api.nvim_buf_is_valid(bufnr) then
     return
   end