MDC button trailing icon support

[gugu]

Now MDC-web supports trailing icon

https://github.com/material-components/material-components-web/tree/master/packages/mdc-button#trailing-icon

[codecov-io]

Codecov Report

Merging #622 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #622      +/-   ##
=========================================
+ Coverage   94.49%   94.5%   +0.01%     
=========================================
  Files          61      61              
  Lines        2560    2565       +5     
  Branches      371     375       +4     
=========================================
+ Hits         2419    2424       +5     
  Misses         50      50              
  Partials       91      91

Impacted Files	Coverage Δ
packages/button/index.tsx	93.33% <100%> (+1.33%)	⬆️

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[moog16]

this also requires unit tests

[gugu]

I made these changes and added some simple test. This morning I'll check if there are more tests needed. If yes - I will add them. And retest module in our application

[moog16]

Also please add one screenshot test for button

[moog16]

Left some comments, but I think once those are addressed it should be enough.

[gugu]

Done

[moog16]

@gugu please update button golden to the same as this: #625

[gugu]

Done

[moog16]

#625 tests passing...looks good!