timers-browserify dependency on setimmediate pollutes global environment

[rjgotten]

timers-browserify depends on setimmediate to make available setImmediate and clearImmediate functions.

This breaks proper encapsulation in Webpack builds where we don't want to pollute the global environment with polyfills. E.g. when authoring drop-in widgets for use within someone else's website and acting as a third-party.

[jryans]

Thanks for the feedback! Feel free to propose a fix via PR... From past experience, someone somewhere seems to depend on subtle details of the current implementation, so I can't make any guarantees that a change would stick.