[resources] add icons sketch library

[estevesd]

Add Sketch Library for icons

Changes proposed in this pull request:

Add a separate Sketch Library for Icons.
16/20px, dark/Light versions are available for easy of use in sketch
Nested components allow overrides with shared styles.
Icons are organised by categories like on the docs

Reviewers should focus on:

Is it useful for everyone? or only me?

Screenshot

[palantirtech]

Thanks for your interest in palantir/blueprint, @estevesd! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[estevesd]

based on discussions in #2817
the icons have been added in the kit instead of a separate library.
shall we keep it like this or still do a separate library only for icons ?

[aycai]

I think for now keeping it in the kit file (as is) works. I'm wary of perf, but it seems to work fine so far.

I was going to say that you don't need separate symbols for dark and light, but checking your file in #2817 looks like you fixed that :)

Changes in there look good as well 👍