feat: dev mode for rollup

[tivac]

Fixes #516 (for rollup at least, more bundlers later if folks want 'em!)

This adds a new option to @modular-css/rollup, dev. Enabling dev mode will make the default exports of each CSS file a JS Proxy that throws whenever a non-existent property is read. Should be much easier to spot the VERY LOUD REFERENCE ERRORS vs trying to find instances of class="undefined", right?

That's the theory anyways!

This also fixes the rollup peerDependency so it stops complaining about rollup 0.66.0 and up.

[codecov]

Codecov Report

Merging #518 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #518      +/-   ##
==========================================
+ Coverage   98.84%   98.84%   +<.01%     
==========================================
  Files          39       39              
  Lines         867      869       +2     
  Branches      131      131              
==========================================
+ Hits          857      859       +2     
  Misses         10       10

Impacted Files	Coverage Δ
packages/rollup/rollup.js	98.85% <100%> (+0.02%)	⬆️

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