src: use single ObjectTemplate for TextDecoder

`ObjectTemplate`s are not garbage-collected like regular objects
(for some reason). It is sufficient to create a single template
anyway, so do that to address the memory leak.

Fixes: https://github.com/nodejs/node/issues/32424

PR-URL: https://github.com/nodejs/node/pull/32426


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>