matchparen: do not use hard-coded match id (vim/vim#13393)
* matchparen: do not use hard-coded match id
Instead of using the hard-coded match id 3, which may also be used by
other plugins, let the matchparen plugin use whatever ids are
automatically returned when calling matchaddpos().
For backwards-compatibility, keep the `:3match` call, which will still
use the hard-coded id 3 (as mentioned in :h :3match).
closes: vim/vim#13381d3e277f279
Co-authored-by: Christian Brabandt <cb@256bit.org>
fc02908c97
