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`.

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.

Problem:
- API functions using `try_start` directly instead of `TRY_WRAP`, do not
  surface the underlying error message, and instead show generic things
  like "Failed to set buffer".
- Error handling code is duplicated in the API impl, instead of
  delegating to the vim buffer/window handling logic.

Solution:
- Use `TRY_WRAP`.

Fix regression from #31195

Problem:  Documentation for "hl_group" in nvim_buf_set_extmark() is
          unclear. "hl_group" in nvim_echo() does not accept
          highlight group id.
Solution: Move documentation for highlight group name/id to first
          mention of hl_group. Update nvim_echo() to accept highlight
          group id.

Problem:  Highlight group id is not propagated to the end of the message call
          stack, where ext_messages are emitted.
Solution: Refactor message functions to pass along highlight group id
          instead of attr id.

Problem:  too many strlen() calls in register.c
Solution: refactor code, add string_T struct to keep track
          of string lengths (John Marriott)

closes: vim/vim#15952

https://github.com/vim/vim/commit/79f6ffd388299ef3b1c95cbe658785e6e66df144

Co-authored-by: John Marriott <basilisk@internode.on.net>

In the api_info() output:

    :new|put =map(filter(api_info().functions, '!has_key(v:val,''deprecated_since'')'), 'v:val')
    ...

    {'return_type': 'ArrayOf(Integer, 2)', 'name': 'nvim_win_get_position', 'method': v:true, 'parameters': [['Window', 'window']], 'since': 1}

The `ArrayOf(Integer, 2)` return type didn't break clients when we added
it, which is evidence that clients don't use the `return_type` field,
thus renaming Dictionary => Dict in api_info() is not (in practice)
a breaking change.