feat(icon-toggle): Added support for SVG icons

[MichaelTamm]

No description provided.

[codecov-io]

Codecov Report

Merging #1898 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1898      +/-   ##
==========================================
+ Coverage   99.43%   99.43%   +<.01%     
==========================================
  Files          84       84              
  Lines        3721     3728       +7     
  Branches      484      486       +2     
==========================================
+ Hits         3700     3707       +7     
  Misses         21       21

Impacted Files	Coverage Δ
packages/mdc-icon-toggle/foundation.js	100% <100%> (ø)	⬆️
packages/mdc-icon-toggle/index.js	100% <100%> (ø)	⬆️

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[flundermicha]

Ping

[MichaelTamm]

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[kfranqueiro]

Sorry for letting this PR lapse for so long. This has now become obsoleted by the mdc-icon-button package (see #2748), which now has basic SVG support.

However, I think this also attempted to implement the ability to support different SVGs for on/off states, which Icon Button currently doesn't. This has been identified in #3107 with a proposed alternative approach to the component's DOM structure.