Decouple the BTR process from the Webpack Plugin

[agubler]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:

In order to run build time rendering independently of the webpack build, the main process needs to be decoupled from the webpack apply API and called directly. In addition this adds a new API that enables a single page/path to be executed.

Related to dojo/cli-build-app#367

[codecov]

Codecov Report

Merging #242 into master will decrease coverage by 0.72%.
The diff coverage is 83.59%.

@@            Coverage Diff             @@
##           master     #242      +/-   ##
==========================================
- Coverage   83.86%   83.14%   -0.73%     
==========================================
  Files          46       46              
  Lines        2114     2142      +28     
  Branches      566      571       +5     
==========================================
+ Hits         1773     1781       +8     
- Misses        341      361      +20

Impacted Files	Coverage Δ
src/build-time-render/BuildTimeRender.ts	90.08% <83.59%> (-5.01%)	⬇️
src/build-time-render/Renderer.ts	92.64% <0%> (+0.1%)	⬆️

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