Expose `JS_DupValueExt`

This commit exposes an API to access JS_DupValue.

I also formatted a bit the value.c file.

[x] I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-core do not require updating CHANGELOG files.
[x] I've updated the relevant crate versions if necessary. Versioning policy for library crates
[x] I've updated documentation including crate documentation if necessary.

bytecodealliance / javy