timers: minor refinements to TimersList & stricter undefined/null checks

A few small refinements to TimersList & ImmediateList:

• Eliminate the createTimersList helper function and instead do all instantiation directly within the constructor
• Define TIMEOUT_MAX as 2 ** 31 - 1 instead of having a comment explaining it
• No longer use delete on unrefed & refed lists as it's expensive and offers no benefit since we never iterate over the dictionary object in regular usage, except in cases where user-code throws and we can likely accept the minute difference in performance in those cases
• Stricter comparisons to null & undefined

Checklist

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

Affected core subsystem(s)

timers