Add CSSFunctions to CSSProperties typings

[quicksnap]

What:

Improves TypeScript support for CSS functions

• [ ] Documentation
• [ ] Tests
• [x] Ready to be merged
• [ ] Added myself to contributors table

[codecov-io]

Codecov Report

Merging #392 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #392   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          10     10           
  Lines         177    177           
  Branches       50     50           
=====================================
  Hits          177    177

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[quicksnap]

@luke-john I've added a TypeScript test for this PR.

Let's open separate tickets for the remaining items. If no one picks them up I'll likely take care of them eventually =)

[quicksnap]

Opened #394 for that one.

[quicksnap]

Huh.. I failed the build but I'm not sure why..

[quicksnap]

That same command (npm run validate) passes on my machine.

[kentcdodds]

My guess is there's a transitive dep issue.... May require a kcd-scripts update... I'll look into it when I find time.