- The previous commit lost information in the tests. Instead, add some
  more "normalization" substitutions in pcall_err(), so that the general
  shape of the stacktrace is included in the asserted text.
- Eliminate contains(), it is redundant with matches()

Can be reproduced with a script like this:

-- in some lua file
vim.fn.timer_start(10, function() error("uh....") end)

-- will cause neovim to crash with the following error.
PANIC: unprotected error in call to Lua API
        (nlua_CFunction_func_call failed.)

After this, it will instead print the error message
from the top of the stack, like so.

tmp/error_nvim.lua:10: uh...

Also added an example test. Previously this test
caused the embedded nvim to panic.

vim-patch:8.2.1054: not so easy to pass a lua function to Vim
vim-patch:8.2.1084: Lua: registering function has useless code

I think I have also opened up the possibility for people to use these
callbacks elsewhere, since I've added a new struct that we should be
able to use.

Also, this should allow us to determine what the state of a list is in
Lua or a dictionary in Lua, since we now can track the luaref as we go.

Also simplify error messages when calling lua from vimL.

- Return VimL errors instead of generic errors for:
  - nvim_call_function
  - nvim_call_dict_function
- Fix tests which were silently broken before this change.

This violates #6150 where we agreed not to translate API errors.  But
that can be fixed later.

Missing: updates to various lists.