Added icon component

[ritave]

Part of https://github.com/MetaMask/MetaMask-planning/issues/1615.

This adds an icon component on the Snaps SDK side of things, awaiting implementation in metamask-extension.

It's usable in Box, Tooltip and Text

[eriknson]

Let's just expose md (20px) size, and default, primary, and muted colors to begin with

[eriknson]

We'll make these—and any other static component—hoverable & clickable using some sort of <onClick> wrapper soon

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.27%. Comparing base (6a8b17b) to head (5eb2bc9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2638      +/-   ##
==========================================
+ Coverage   94.18%   94.27%   +0.09%     
==========================================
  Files         477      478       +1     
  Lines        9834     9999     +165     
  Branches     1498     1499       +1     
==========================================
+ Hits         9262     9427     +165     
  Misses        572      572              

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