refactor(fs): replace vim.fn/vim.env in vim.fs (#20379)

Avoid using vim.env and vim.fn in vim.fs functions so that
they can be used in "fast" contexts.

runtime/lua/vim/fs.lua

@@ -1,5 +1,7 @@
 local M = {}
 
+local iswin = vim.loop.os_uname().sysname == 'Windows_NT'
+
 --- Iterate over all the parents of the given file or directory.
 ---
 --- Example:
@@ -40,7 +42,19 @@ function M.dirname(file)
   if file == nil then
     return nil
   end
-  return vim.fn.fnamemodify(file, ':h')
+  vim.validate({ file = { file, 's' } })
+  if iswin and file:match('^%w:[\\/]?$') then
+    return (file:gsub('\\', '/'))
+  elseif not file:match('[\\/]') then
+    return '.'
+  elseif file == '/' or file:match('^/[^/]+$') then
+    return '/'
+  end
+  local dir = file:match('[/\\]$') and file:sub(1, #file - 1) or file:match('^([/\\]?.+)[/\\]')
+  if iswin and dir:match('^%w:$') then
+    return dir .. '/'
+  end
+  return (dir:gsub('\\', '/'))
 end
 
 --- Return the basename of the given file or directory
@@ -48,7 +62,14 @@ end
 ---@param file (string) File or directory
 ---@return (string) Basename of {file}
 function M.basename(file)
-  return vim.fn.fnamemodify(file, ':t')
+  if file == nil then
+    return nil
+  end
+  vim.validate({ file = { file, 's' } })
+  if iswin and file:match('^%w:[\\/]?$') then
+    return ''
+  end
+  return file:match('[/\\]$') and '' or (file:match('[^\\/]*$'):gsub('\\', '/'))
 end
 
 --- Return an iterator over the files and directories located in {path}
@@ -229,7 +250,12 @@ end
 ---@return (string) Normalized path
 function M.normalize(path)
   vim.validate({ path = { path, 's' } })
-  return (path:gsub('^~/', vim.env.HOME .. '/'):gsub('%$([%w_]+)', vim.env):gsub('\\', '/'))
+  return (
+    path
+      :gsub('^~/', vim.loop.os_homedir() .. '/')
+      :gsub('%$([%w_]+)', vim.loop.os_getenv)
+      :gsub('\\', '/')
+  )
 end
 
 return M