Fix: Ensure all css is written and deduped correctly in BTR

[agubler]

Type: bug

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:

The existing BTR uses some rudimentary logic to try and avoid writing duplicate CSS to the index.html file(s). However there are multiple edge cases where this is not good enough and we end up not writing CSS selectors that are required.

This change removes this logic and replaces it will a post processing phase using postcss and cssnano to correctly optimise the styles prior to writing them to the index.html

[codecov]

Codecov Report

Merging #106 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
- Coverage   84.05%   84.02%   -0.03%     
==========================================
  Files          37       37              
  Lines        1392     1390       -2     
  Branches      373      372       -1     
==========================================
- Hits         1170     1168       -2     
  Misses        222      222

Impacted Files	Coverage Δ
src/build-time-render/BuildTimeRender.ts	98.15% <100%> (-0.03%)	⬇️

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