Problem:    Sourcing buffer lines is too complicated.
Solution:   Simplify the code. Make it possible to source Vim9 script lines.
            (Yegappan Lakshmanan, closes vim/vim#9974)

https://github.com/vim/vim/commit/85b43c6cb7d56919e245622f4e42db6d8bee4194

This commit changes the behavior of sourcing buffer lines to always have
a script ID, although sourcing the same buffer always produces the same
script ID.

vim-patch:9.1.0372: Calling CLEAR_FIELD() on the same struct twice

Problem:  Calling CLEAR_FIELD() on the same struct twice.
Solution: Remove the second CLEAR_FIELD().  Move the assignment of
          cookie.sourceing_lnum (zeertzjq).

closes: vim/vim#14627

https://github.com/vim/vim/commit/f68517c1671dfedcc1555da50bc0b3de6d2842f6

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

Problem:
`vim.tbl_get(tbl, nil, 1)` returns `tbl` itself. In this case, `keys` is not
empty, but `ipairs` skips the iteration:

    local keys = { nil, 1 }
    assert(#keys == 2)
    for i, k in ipairs(keys) do
      assert(false, 'unreachable')
    end

Solution:
Use `select("#", ...)` and `select(i, ...)` to ensure consistency for count and
iteration.

Problem:
- API functions using `try_start` directly, do not surface the
  underlying error message, and instead show generic messages.
- Error-handling code is duplicated in the API impl.
- Failure modes are not tested.

Solution:
- Use `TRY_WRAP`.
- Add tests.

Problem:  Lua callbacks for "msg_show" events with vim.ui_attach() are
          executed when it is not safe.
Solution: Disallow non-fast API calls for "msg_show" event callbacks.
          Automatically detach callback after excessive errors.
          Make sure fast APIs do not modify Nvim state.

Before calling "attach" a screen object is just a dummy container for
(row, col) values whose purpose is to be sent as part of the "attach"
function call anyway.

Just create the screen in an attached state directly. Keep the complete
(row, col, options) config together. It is still completely valid to
later detach and re-attach as needed, including to another session.

PROBLEM:
There are several limitations to vim.str_byteindex, vim.str_utfindex:
1. They throw given out-of-range indexes. An invalid (often user/lsp-provided)
   index doesn't feel exceptional and should be handled by the caller.
   `:help dev-error-patterns` suggests that `retval, errmsg` is the preferred
   way to handle this kind of failure.
2. They cannot accept an encoding. So LSP needs wrapper functions. #25272
3. The current signatures are not extensible.
    * Calling: The function currently uses a fairly opaque boolean value to
      indicate to identify the encoding.
    * Returns: The fact it can throw requires wrapping in pcall.
4. The current name doesn't follow suggestions in `:h dev-naming` and I think
   `get` would be suitable.

SOLUTION:
- Because these are performance-sensitive, don't introduce `opts`.
- Introduce an "overload" that accepts `encoding:string` and
  `strict_indexing:bool` params.

```lua
local col = vim.str_utfindex(line, encoding, [index, [no_out_of_range]])
```

Support the old versions by dispatching on the type of argument 2, and
deprecate that form.

```lua
vim.str_utfindex(line)                             -- (utf-32 length, utf-16 length), deprecated
vim.str_utfindex(line, index)                      -- (utf-32 index, utf-16 index), deprecated
vim.str_utfindex(line, 'utf-16')                   -- utf-16 length
vim.str_utfindex(line, 'utf-16', index)            -- utf-16 index
vim.str_utfindex(line, 'utf-16', math.huge)        -- error: index out of range
vim.str_utfindex(line, 'utf-16', math.huge, false) -- utf-16 length
```

Problem:

`vim.validate()` takes two forms when it only needs one.

Solution:

- Teach the fast form all the features of the spec form.
- Deprecate the spec form.
- General optimizations for both forms.
- Add a `message` argument which can be used alongside or in place
  of the `optional` argument.