fix(fs): make normalize() work with '/' path (#24047)

Problem: Current implementation of "remove trailing /" doesn't
account for the case of literal '/' as path.
Solution: Remove trailing / only if it preceded by something else.

Co-authored by: notomo <notomo.motono@gmail.com>

runtime/lua/vim/fs.lua

@@ -348,9 +348,7 @@ function M.normalize(path, opts)
     path = path:gsub('%$([%w_]+)', vim.uv.os_getenv)
   end
 
-  path = path:gsub('\\', '/'):gsub('/+', '/')
-
-  return path:sub(-1) == '/' and path:sub(1, -2) or path
+  return (path:gsub('\\', '/'):gsub('/+', '/'):gsub('(.)/$', '%1'))
 end
 
 return M