inspector: make sure timer handles are cleaned up

It is not obvious that timer handles are cleaned up properly when the current Environment exits, nor that the Environment knows to keep track of the closing handles.

This change may not be necessary, because timer handles close without non-trivial delay (i.e. at the end of the current event loop term), and JS-based inspector sessions (which are the only ones we can easily test) are destroyed when cleaning up, closing the timers as a result. I don’t know what happens for other kinds of inspector sessions, though.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines