Support for On Demand BTR

[agubler]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier
• Unit or Functional tests are included in the PR
• schema.json has been updated appropriately

Description:

Uses the on demand BTR middleware from webpack-contrib and turns the mode on when using btr in build and watch.

Resolves #367

[codecov]

Codecov Report

Merging #388 into master will decrease coverage by 0.19%.
The diff coverage is 82.50%.

@@            Coverage Diff             @@
##           master     #388      +/-   ##
==========================================
- Coverage   48.51%   48.32%   -0.20%     
==========================================
  Files          10       10              
  Lines         707      716       +9     
  Branches      206      213       +7     
==========================================
+ Hits          343      346       +3     
- Misses        364      370       +6     

Impacted Files	Coverage Δ
src/base.config.ts	19.24% <0.00%> (ø)
src/dev.config.ts	18.33% <0.00%> (-0.32%)	⬇️
src/dist.config.ts	27.27% <0.00%> (-0.42%)	⬇️
src/main.ts	87.50% <89.18%> (-1.69%)	⬇️

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