Add meetup exercise

[ageron]

Sadly the roc-isodate package does not provide a way to know a date's weekday, or to figure out how many days a month has. I've submitted a PR and an issue for that. Also, even though the roc-isodate package defines the isLeapYear function, it's not exposed, I've asked for that function to be exposed.

So perhaps we should wait until it's merged and a new version is released and the test runner is updated to use that version before launching this exercise, otherwise a large part of the effort is in implementing these functions, and it looks bad for Roc.
On the other hand I had fun implementing these functions... Wdyt?

[Anton-4]

perhaps we should wait until it's merged and a new version is released and the test runner is updated to use that version before launching this exercise

I would indeed prefer this

[imclerran]

Saw your PR - I'll work on adding the requested features this week.

[imclerran]

Alright, all the requested functions have been added and released in v0.5.1!

[ageron]

Cool, thanks @imclerran , I updated this exercise to use isodate version 0.5.1, the solution is much shorter now that I can just import weekday and dayOfMonth. 👍
Note: before we merge this PR, we must wait until the roc-test-runner is upgraded to support isodate 0.5.1 (see exercism/roc-test-runner#16)

[isaacvando]

LGTM!