Deëxperiment some recent features in time for 5.40

[leonerd]

We recently decided that these four features seem stable enough to remove experimental warnings from.

Three features removed warnings entirely:

• sub :const
• foreach my (LIST...)
• extra paired delimiters

Additionally, removed experimental warnings from try/catch blocks that don't have a finally on them. The finally itself remains experimental due to complications about exception-during-exception, for the same reason that defer does.

[leonerd]

One thing I specifically did not do, is add the now-stable features (try, extra_paired_delimiters) to the v5.40 feature bundle. It seems a bit too close to release now to consider adding those, in case that might cause extra complications. Hopefully we'll consider adding them to 5.42 though.

[Leont]

Please do follow up with a experimental.pm PR