fix: ref() and unref() return this to align with NodeJS Timeout api

[TillaTheHun0]

Thank you for your work on this module!

Background

NodeJS' Timeout.ref() and Timeout.unref() both return a reference to the Timeout.

This allows for doing things like:

const timeout = setTimeout(() => {}, 100).unref()

// later
clearTimeout(timeout)

instead of having to call timeout.unref() as a separate expression.

Problem

Since long-timeout's impls of ref() and unref() return undefined, the above usage will result in unexpected behavior -- the timeout is never cleared and will later be invoked (clearTimeout will simply noop, if passed anything that is not a timeout)

Changes

This PR updates ref() and unref() on Timeout and Interval to return this, such to match NodeJS' api and get long-timeout closer to a drop-in replacement (at least w.r.t ref() and unref())