Support evergreen browsers

[matt-gadd]

Adds a new legacy flag that defaults to true to be backwards compatible. When set to false, uses our esm versions (.mjs) of our deps rather than the down emitted es5 umd (.js) modules.

Has to use a custom ts import transformer (which we should eventually move to webpack-contrib), to give us the same parity on bundling/codesplitting that we got before from a combination of the promise-loader and umd-compat-loader, neither which are used in evergreen.

Apart from the above the changes are mostly cribbed from the cli-build-widget changes here: dojo/cli-build-widget#11

[codecov]

Codecov Report

Merging #100 into master will decrease coverage by 4.63%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
- Coverage   68.07%   63.43%   -4.64%     
==========================================
  Files           8        8              
  Lines         332      361      +29     
  Branches       59       67       +8     
==========================================
+ Hits          226      229       +3     
- Misses        101      127      +26     
  Partials        5        5

Impacted Files	Coverage Δ
src/main.ts	98.16% <100%> (+0.01%)	⬆️
src/base.config.ts	26.59% <6.25%> (-8.26%)	⬇️

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