Move to postcss-preset-env over postcss-cssnext

[agubler]

postcss-cssnext has been deprecated, so move to the replacement postcss-present-env and only include features required for IE11 if legacy is true.

Related to dojo/meta#262

[codecov]

Codecov Report

Merging #139 into v3 will decrease coverage by 0.19%.
The diff coverage is 33.33%.

@@            Coverage Diff            @@
##               v3     #139     +/-   ##
=========================================
- Coverage   60.53%   60.33%   -0.2%     
=========================================
  Files           8        8             
  Lines         413      416      +3     
  Branches       92       94      +2     
=========================================
+ Hits          250      251      +1     
- Misses        163      165      +2

Impacted Files	Coverage Δ
src/base.config.ts	28.28% <33.33%> (+0.15%)	⬆️

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