[navigator] Migrate to R2 JS

[jmgeffroy]

There are JS sources in

• kotlin-toolkit (https://github.com/readium/kotlin-toolkit/tree/main/readium/navigator/src/main/assets/_scripts/src),
• swift-toolkit (https://github.com/readium/swift-toolkit/tree/main/Sources/Navigator/EPUB/Scripts/src) Only a few differences. These must obviously be unified for our Flutter version.

[jmgeffroy]

For Android, fixed with commits 156b9a373a877deb810e7afd690f547497d232a9 and 02985653ff9a07136e8782a1960fcb3b36ba9bd1

Not tested on iOS yet ==> TBD: unify JS with the Swift Toolkit version of R2 JS.