Problem:  Combined highlighting was not applied to nvim_eval_statusline(),
          and 'statuscolumn' sign segment/numhl highlights.
Solution: Add an additional `groups` element to the return value of
          `nvim_eval_statusline()->highlights`. This is an array of stacked
          highlight groups (highest priority last). Also resolve combined
          highlights for the 'statuscolumn' sign segment/numhl highlights.
          Expose/synchronize some drawline.c logic that is now mimicked in
          three different places.

Problem:  No "verbose" kind for nvim_echo() opts->verbose.
Solution: Pass NULL "kind" to indicate no new kind.

Problem:
The `nvim_notify` API (note: unrelated to `vim.notify()` Lua API) was
not given any real motivation in https://github.com/neovim/neovim/pull/13843

There are, and were, idiomatic and ergonomic alternatives already.

Solution:
Deprecate `nvim_notify`.

Problem:  We want to deprecate `nvim_err_write(ln)()` but there is no
          obvious replacement (from Lua). Meanwhile we already have
          `nvim_echo()` with an `opts` argument.
Solution: Add `err` argument to `nvim_echo()` that directly maps to
          `:echoerr`.

Problem:
If completeopt does not include "popup" flag, nvim__complete_set still
auto-creates a floating preview window.

Solution:
Fail if completeopt does not include the "popup" flag.

Problem:
`nvim__redraw()` doesn't clamp the lines in the `range` parameter before truncating to int. The resulting range may be empty when the original range contained buffer lines and vice versa.

E.g. for a buffer with 4 lines, these are the redrawn lines:

```lua
{ 2, 2 ^ 31 } -> none (should be { 2, 3 })
{ 2, 2 ^ 32 } -> none (should be { 2, 3 })
{ 2 ^ 32 - 1, 2 } -> { 0, 1 } (should be none)
```

Solution:
Clamp `range` values before truncating to int.

Result of `make iwyu` (after some "fixups").

Problem:  Two separate try/end wrappers, that only marginally differ by
          restoring a few variables. Wrappers that don't restore
          previous state are dangerous to use in "api-fast" functions.
Solution: Remove wrappers that don't restore the previous state.
          Always use TRY_WRAP.