Add single bundle mode for the build output

[agubler]

Adds an options to produce a single "main" bundle in the build output.

[mwistrand]

The phrase "single file" is not clear. Perhaps something like "single bundle" would be better?

[codecov]

Codecov Report

Merging #145 into master will decrease coverage by 1.16%.
The diff coverage is 9.09%.

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
- Coverage   58.05%   56.89%   -1.17%     
==========================================
  Files           8        8              
  Lines         453      464      +11     
  Branches      102      112      +10     
==========================================
+ Hits          263      264       +1     
- Misses        190      200      +10

Impacted Files	Coverage Δ
src/dev.config.ts	24.52% <0%> (-2.01%)	⬇️
src/dist.config.ts	32.2% <0%> (-2.35%)	⬇️
src/base.config.ts	28.2% <0%> (-0.5%)	⬇️
src/main.ts	100% <100%> (ø)	⬆️

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