[Rd] R and maintenance of TRE library

[Kevin Ushey]

Hi,

It's my understanding that, for some time, TRE has been unmaintained
upstream, and so R Core has been maintaining their own copy with
patches locally. Fortunately, upstream
(https://github.com/laurikari/tre) has seen renewed activity; e.g. a
0.9.0 release in September 2024 (the first since 0.8.0 in 2009) and
has had fixes merged through 2026.

I'm planning to explore how far R's own copy of TRE has diverged from
upstream, and see if we can reconcile the changes (e.g. generating
pull requests incorporating any necessary changes that R has made).
This work will happen independently from R, but hopefully will ease
the pathway towards consuming TRE from upstream again in the future.

Thanks,
Kevin