fix: allow multiline message for echoerr (#25380)

PROBLEM:

Currently `:echoerr` prints multi-line strings in a single line
as `:echom` does (Note: `:echon` can print multi-line strings well).
This makes stacktrace printed via echoerr difficult to read.

Example code:

    try
      lua error("lua stacktrace")
    catch
      echoerr v:exception
    endtry

Output:

    Error detected while processing a.vim[5]..a.vim:
    line    4:
    Vim(lua):E5108: Error executing lua [string ":lua"]:1: lua stacktrace^@stack traceback:^@^I[C]: in function 'error'^@^I[string ":lua"]:1: in main chunk

SOLUTION:

Allow echoerr to print multiline messages (e.g., lua exceptions),
because this command is usually used to print stacktraces.

Output after the fix:

    Error detected while processing a.vim[5]..a.vim:
    line    4:
    Vim(lua):E5108: Error executing lua [string ":lua"]:1: lua stacktrace
    stack traceback:
            [C]: in function 'error'
            [string ":lua"]:1: in main chunk

src/nvim/eval.c

@@ -8106,7 +8106,7 @@ void ex_execute(exarg_T *eap)
       // We don't want to abort following commands, restore did_emsg.
       int save_did_emsg = did_emsg;
       msg_ext_set_kind("echoerr");
-      emsg(ga.ga_data);
+      emsg_multiline(ga.ga_data, true);
       if (!force_abort) {
         did_emsg = save_did_emsg;
       }