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/eval/funcs.c

@@ -1716,7 +1716,7 @@ static void f_expand(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
       emsg_off++;
     }
     size_t len;
-    char *errormsg = NULL;
+    const char *errormsg = NULL;
     char *result = eval_vars((char *)s, s, &len, NULL, &errormsg, NULL, false);
     if (p_verbose == 0) {
       emsg_off--;
@@ -1781,7 +1781,7 @@ static void f_menu_get(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 /// Expand all the special characters in a command string.
 static void f_expandcmd(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 {
-  char *errormsg = NULL;
+  const char *errormsg = NULL;
   bool emsgoff = true;
 
   if (argvars[1].v_type == VAR_DICT