Extracting shared resources out to a bootstrap file

[rorticus]

Currently, each custom element bundle contains a copy of the vdom. This is inefficient and bloats the bundle size.

This PR extracts anything used more than twice from node_modules into a common bootstrap chunk. This bootstrap chunk will contain the dojo framework and other modules shared between your elements and will only need to be loaded once.

What this means is:

• If you build more than one element, a bootstrap js file will be generated. You'll need to load this file in addition to your element file.
• If you only build one element, no bootstrap file will be generated.

Note: This is a breaking change, as it modifies the generated chunks

[codecov]

Codecov Report

Merging #95 (578e0f4) into master (3c588a2) will decrease coverage by 0.11%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
- Coverage   57.00%   56.88%   -0.12%     
==========================================
  Files           9        9              
  Lines         500      501       +1     
  Branches      122      122              
==========================================
  Hits          285      285              
- Misses        215      216       +1     

Impacted Files	Coverage Δ
src/base.config.ts	19.01% <0.00%> (-0.14%)	⬇️

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