fix(api): nvim_parse_cmd error message in pcall() #23297

Problem:
nvim_parse_cmd() in pcall() may show an error message (side-effect):

    :lua pcall(vim.api.nvim_parse_cmd, vim.fn.getcmdline(), {})
    E16: Invalid range

Solution:
Avoid emsg() in the nvim_parse_cmd() codepath.

- refactor(api): add error message output parameter to get_address()
- fix: null check emsg() parameter
- refactor: remove emsg_off workaround from do_incsearch_highlighting()
- refactor: remove emsg_off workaround from cmdpreview_may_show()
- refactor: remove remaining calls to emsg() from parse_cmd_address() and get_address()
- (refactor): lint set_cmd_dflall_range()
- refactor: addr_error() - move output parameter to return value

Fix #20339

TODO:

These are the functions called by `get_address()`:

```
nvim_parse_cmd() -> parse_cmdline() -> parse_cmd_address() -> get_address()
    skipwhite()
    addr_error()
    qf_get_cur_idx()
    qf_get_cur_valid_idx()
    qf_get_size()
    qf_get_valid_size()
    mark_get()
    mark_check()
    assert()
    skip_regexp()
    magic_isset()
>   do_search()
>   searchit()
    ascii_isdigit()
    getdigits()
    getdigits_int32()
    compute_buffer_local_count()
    hasFolding()
```

From these functions, I found at least two that call emsg directly:
- do_search()
  - seems to be simple to refactor
- searchit()
  - will be more challenging because it may generate multiple error messages,
    which can't be handled by the current `errormsg` out-parameter.
    For example, it makes multiple calls to `vim_regexec_multi()` in a loop that
    possibly generate error messages, and later `searchit()` itself may generate
    another one:
    - c194acbfc4/src/nvim/search.c (L631-L647)
    - c194acbfc4/src/nvim/search.c (L939-L954)

---------

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

src/nvim/api/command.c

@@ -109,7 +109,7 @@ Dictionary nvim_parse_cmd(String str, Dictionary opts, Error *err)
   exarg_T ea;
   CmdParseInfo cmdinfo;
   char *cmdline = string_to_cstr(str);
-  char *errormsg = NULL;
+  const char *errormsg = NULL;
 
   if (!parse_cmdline(cmdline, &ea, &cmdinfo, &errormsg)) {
     if (errormsg != NULL) {