matchparen: do not use hard-coded match id (#13393)

matchparen: do not use hard-coded match id (#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: #13381

Signed-off-by: Christian Brabandt <cb@256bit.org>