Correct units and cumulative bug in GC times (#9890)

Fixes #7080

In the logging page, GC events were displayed with summaries like idle
collection in 290 ms. The value 290 was calculated directly from the cumulative
times (`newSpace.time` and `oldSpace.time`), meaning it represented the total
cumulative GC time since isolate startup rather than the duration of this
specific GC. In reality, the individual collection took 290 microseconds (which
is 0.29 milliseconds), but since it was displaying the cumulative time of 290
ms, it was off by a factor of 1000x.

In this fix, we update `_handleGCEvent` to track and subtract the previous
cumulative GC time from the new cumulative GC time to compute the actual
duration delta for this specific GC.

Also format the duration as a millisecond double with 1 decimal place
(consistent with how frame times are displayed), and appended it to the log
summary (e.g. "idle collection in 100.0 ms").