cli: rejigger --version to include PCRE2 info

This adds info about whether PCRE2 is available or not to the output of
--version. Essentially, --version now subsumes --pcre2-version, although
we do retain the former because it (usefully) emits an exit code based
on whether PCRE2 is available or not.

Closes #2645

crates/core/flags/mod.rs

@@ -30,6 +30,7 @@ pub(crate) use crate::flags::{
         man::generate as generate_man_page,
         version::{
             generate_long as generate_version_long,
+            generate_pcre2 as generate_version_pcre2,
             generate_short as generate_version_short,
         },
     },