timers-browserify dependency on setimmediate pollutes global environment

`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.