Adding support for custom element transformer

[rorticus]

Adding support for the new custom element transformer.

Notable changes:

• Upgrading to @dojo/framework 6.0.0-alpha to get changes to registerCustomElement
• Upgrading ts-loader because we need at least 5.3.3 (includes support for sending program instance to custom transformers)
• Turning off onlyCompileBundledFiles so that all type information is sent to custom transformers.
• Removed test files from test-app/ that weren't being used
• Removed @customElement decorators from test-app/ widgets, relying on new custom element transformer instead.

Relates to: dojo/webpack-contrib#43

[rorticus]

Need to bump ts-loader. 5.3.3 makes the typescript program available to the transformers.

[rorticus]

Note that CI isn't going to pass as it's dependent on dojo/webpack-contrib#133

[codecov]

Codecov Report

Merging #53 into master will decrease coverage by 0.71%.
The diff coverage is 20%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage    71.3%   70.58%   -0.72%     
==========================================
  Files           8        8              
  Lines         352      357       +5     
  Branches       59       60       +1     
==========================================
+ Hits          251      252       +1     
- Misses        101      105       +4

Impacted Files	Coverage Δ
src/base.config.ts	33.82% <20%> (-1.1%)	⬇️

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[codecov]

Codecov Report

Merging #53 into master will decrease coverage by 0.63%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage    71.3%   70.67%   -0.64%     
==========================================
  Files           8        8              
  Lines         352      358       +6     
  Branches       59       59              
==========================================
+ Hits          251      253       +2     
- Misses        101      105       +4

Impacted Files	Coverage Δ
src/main.ts	95.27% <100%> (+0.03%)	⬆️
src/base.config.ts	33.82% <20%> (-1.1%)	⬇️

---

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