fix(exrc): exrc knows its own location (#34638)

fix(exrc): lua exrc files know their location

Problem:
'exrc' files are inherently bound to their location / workspace and
therefore require to "know" their location on the filesystem. However,
currently using `debug.getinfo(1, 'S')` returns `"<nvim>"`.

Solution:
Include the filepath as chunkname in `loadstring()` and `nlua_exec()`.

runtime/lua/vim/_defaults.lua

@@ -956,7 +956,7 @@ do
         local trusted = vim.secure.read(file) --[[@as string|nil]]
         if trusted then
           if vim.endswith(file, '.lua') then
-            assert(loadstring(trusted))()
+            assert(loadstring(trusted, '@' .. file))()
           else
             vim.api.nvim_exec2(trusted, {})
           end