feat(api): add err field to nvim_echo() opts

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`.

src/nvim/api/keysets_defs.h

@@ -325,6 +325,7 @@ typedef struct {
 } Dict(cmd_opts);
 
 typedef struct {
+  Boolean err;
   Boolean verbose;
 } Dict(echo_opts);