[beta] TRPL: incorporate all backward-compatible Edition changes

This incorporates all the backwards-compatible changes for the 2024
Edition. There will also be a follow-on PR to land revisions to the new
chapter on async so it can be as ready as possible when officially
released with 1.85 and the 2024 Edition.

Additionally, there are a few other, non-backward-compatible, changes
(largely around `use<..>`) we can only land using the stable edition,
which we may or may not be able to land in 1.85 by using the beta
toolchain in the example code. Those may or may not be ported over,
depending on how that does or does not play with the infrastructure.