[libstd_unicode] Expose UnicodeVersion type
In <https://github.com/rust-lang/rust/pull/42998>, we added an
uninstantiable type for the internal `UNICODE_VERSION` value,
`UnicodeVersion`, but it was not made public to the outside of the
crate, resulting in the value becoming less useful. Here we make the
type accessible from the outside.
Also add a run-pass test to make sure the type and value can be accessed
as intended.
23b4020156