Promise: add .finally()

[wmertens]

Fixes #169

Thanks to https://stackoverflow.com/a/53327815/124416

[josdejong]

Thanks Wout!

Interesting to see that the finally propagates both the result or error, I wasn't aware of that.

Two feedbacks:

• I'm wondering if there is need to document this method. I think not, since it's so standard. What do you think?
• Can you write unit tests for this method?

[joshLong145]

@josdejong @wmertens
This will be a useful addition until there can be a migration to the native Promise library. I have added some tests in a new branch to confirm matching functionality with the native finally implementation.

https://github.com/joshLong145/workerpool/tree/feat/add-promise-finally

[wmertens]

@joshLong145 I added your commit to this branch

[josdejong]

Thanks for the update Wout, it has been a while 😅 .

Two remarks:

1. Can you describe the new .finally() method in the README.md? There is a list with all methods there that can be extended.
2. I think the unit tests need to be extended a bit: when I remove the .then and .catch from the two tests, the test still succeeds. So, I think the test should check whether the .then and .catch have been trigered before calling done. It is valuable at least to verify the order in which the .then, .catch, and .finally are triggered so these two tests make sense to me.
3. Similarly, the finally method neatly propagates either the success or failure, but this is not yet captured in a unit test. Can you add two unit tests for that?