timers: allow setTimeout to accept a delay of 0

[mcollina]

Summary

setTimeout() now accepts a delay of 0 (and positive sub-millisecond delays, which are truncated to 0 by insert()), scheduling the callback as soon as possible instead of silently clamping it to 1 ms. This matches browser (Chrome) behavior and resolves the ordering surprise described in #46596.

Negative delays, NaN, and values above TIMEOUT_MAX are still clamped to 1 ms with the existing warnings. setInterval() continues to clamp delays below 1 ms to 1 ms so it does not fire as fast as the event loop allows.

Behavior change

Before:

setTimeout(() => console.log('1st'), 1);
setTimeout(() => console.log('2nd'), 0);
// 1st
// 2nd

After (matches browsers):

setTimeout(() => console.log('1st'), 1);
setTimeout(() => console.log('2nd'), 0);
// 2nd
// 1st

Test plan

• New test test/parallel/test-timers-zero-delay-ordering.js verifies setTimeout(fn, 0) runs before setTimeout(fn, 1) and that timers/promises.setTimeout(0) resolves.
• All 72 test-timers* / mock-timers / promisified tests pass, plus a broader cross-subsystem subset (vm, worker, stream, http, net).

Refs: #46596

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.27%. Comparing base (ebef774) to head (1ddf9b2).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66155      +/-   ##
==========================================
- Coverage   90.28%   90.27%   -0.02%     
==========================================
  Files         790      790              
  Lines      271642   271982     +340     
  Branches    51846    51927      +81     
==========================================
+ Hits       245260   245537     +277     
- Misses      16889    16949      +60     
- Partials     9493     9496       +3     

Files with missing lines	Coverage Δ
lib/internal/timers.js	100.00% <100.00%> (ø)

... and 56 files with indirect coverage changes

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[jasnell]

... as soon as possible

Does this mean at least one event loop turn or within an event loop turn?

[jasnell]

LGTM once CI is green and the "as soon as possible" is clarified.