fix(package): Add source-map files to npm releases

[pavelsevcik]

Fix for missing source-map files in npm releases

Fixes #2942

[codecov-io]

Codecov Report

Merging #4206 into master will increase coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4206      +/-   ##
==========================================
+ Coverage    98.5%   98.84%   +0.33%     
==========================================
  Files         127      127              
  Lines        5629     5629              
  Branches      747      747              
==========================================
+ Hits         5545     5564      +19     
+ Misses         84       65      -19

Impacted Files	Coverage Δ
packages/mdc-checkbox/index.js	100% <0%> (+1.2%)	⬆️
packages/mdc-ripple/index.js	100% <0%> (+2.08%)	⬆️
packages/mdc-auto-init/index.js	100% <0%> (+2.77%)	⬆️
packages/mdc-tab/index.js	96.49% <0%> (+3.5%)	⬆️
packages/mdc-ripple/util.js	100% <0%> (+3.5%)	⬆️
packages/mdc-base/component.js	100% <0%> (+4.16%)	⬆️
packages/mdc-switch/index.js	97.95% <0%> (+6.12%)	⬆️
packages/mdc-tabs/tab/index.js	100% <0%> (+9.09%)	⬆️
packages/mdc-radio/index.js	97.43% <0%> (+12.82%)	⬆️

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[kfranqueiro]

Tested and seems good to me, thanks! Will merge once tests finish after syncing the branch.