JavaScriptEventLoop: Read the clock less often with yieldInterval

[krodak]

Overview

Follow-up to #26, from the review of swiftwasm#821. With yieldInterval set, the drain reads the clock after every job. ContinuousClock.now goes through the WASI shim to performance.now(), and for small jobs that read costs more than the job itself.

This PR reads the clock every N jobs instead. N starts at 1 and doubles (up to 256) while less than a quarter of the interval has passed, so slow jobs are still checked after every job, and long runs of small jobs pay for a clock read only now and then. With yieldInterval == nil nothing changes.

1M chained Task.yield() hops, Node.js, release	nil	yieldInterval set
Clock read after every job (#26)	~135 ns/job	~820 ns/job
This PR	~135 ns/job	~195 ns/job

Trade-off: if a drain switches from many small jobs to slow ones, up to N - 1 slow jobs can run before the next clock read, so the yield can come later than the interval. N goes back to 1 at the start of every drain.

Testing

• make unittest
• Embedded: JavaScriptEventLoop builds with the Embedded SDK.
• Benchmark: 1M chained Task.yield() hops, 3 interleaved runs per build, median of 5 rounds each.
• ./Utilities/format.swift applied.