Auto-drop borrows

[guybedford]

In https://github.com/bytecodealliance/jco/pull/262, we now validate all borrows properly.

The need to call resource[Symbol.dispose]() explicitly for all functions that take resources seems unnecessary though.

Rather, the implementation should always auto-drop borrows at the bindgen level without requiring the JS user to have to do that explicitly. Explicitly drop calls should instead act as early drops if desired.