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`.

runtime/lua/vim/_meta/api_keysets.lua

@@ -88,6 +88,7 @@ error('Cannot require a meta file')
 --- @field pattern? string|string[]
 
 --- @class vim.api.keyset.echo_opts
+--- @field err? boolean
 --- @field verbose? boolean
 
 --- @class vim.api.keyset.empty