Upgrade webpack v4

[mwistrand]

• Update webpack-contrib to next
• Replace ExtractTextPlugin with MiniCssExtractPlugin, but pin to 0.4.2 until danethurber/webpack-manifest-plugin#167 is resolved.
• Replace removed CommonsChunkPlugin with optimize.splitChunks configuration option
• Remove plugins provided by mode configuration option: NamedChunksPlugin, NamedModulesPlugin, and DefinePlugin
• Use terser-webpack-plugin over uglifyjs-webpack-plugin-terser
• Upgrade to webpack-mild-compile@3.3.1
• Force CSS splitting by entry
• Filter css order warnings
• Add optimize-css-assets-webpack-plugin to dist config
• Update tapable typings

Resolves #26

[codecov[bot]]

Codecov Report

Merging #47 into master will decrease coverage by 1.79%. The diff coverage is 31.11%.

@@            Coverage Diff            @@
##           master      #47     +/-   ##
=========================================
- Coverage   71.62%   69.83%   -1.8%     
=========================================
  Files           8        8             
  Lines         356      358      +2     
  Branches       61       62      +1     
=========================================
- Hits          255      250      -5     
- Misses        101      108      +7

Impacted Files	Coverage Δ
src/test.config.ts	14.63% <0%> (-1.65%)	:arrow_down:
src/dev.config.ts	30.76% <0%> (-2.57%)	:arrow_down:
src/base.config.ts	30.43% <37.5%> (-2.38%)	:arrow_down:
src/dist.config.ts	44.44% <50%> (-7.73%)	:arrow_down:
src/main.ts	95.23% <62.5%> (-2.33%)	:arrow_down:

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