fix(lua): use rawget() to get __call in vim.is_callable() (#29536)

Lua 5.1 uses a "raw get" to retrieve `__call` from a metatable to
determine if a table is callable. Mirror this behavior in
`vim.is_callable()`.

runtime/lua/vim/shared.lua

@@ -997,7 +997,7 @@ function vim.is_callable(f)
   if m == nil then
     return false
   end
-  return type(m.__call) == 'function'
+  return type(rawget(m, '__call')) == 'function'
 end
 
 --- Creates a table whose missing keys are provided by {createfn} (like Python's "defaultdict").