Adding "validateConfig" export from webpack plugin

[joelgallant]

Closes #84.

This is nice because it's extremely minimal, rather than importing ajv.

• [x] Cypress test: validate an incorrect config
• [x] Unit test to ensure webpack in 'production' mode can tree shake code generated
• [x] Document example of using query parameter

[cypress[bot]]

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Test summary

5 • 0 • 0 • 0

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Run details

Project	App Config
Status	Passed
Commit	d1db641c86
Started	Jan 10, 2021 3:48 PM
Ended	Jan 10, 2021 3:49 PM
Duration	01:20 💡
OS	Linux Ubuntu - 18.04
Browser	Electron 87

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[codecov-io]

Codecov Report

Merging #81 (d1db641) into master (2ece51b) will decrease coverage by 0.06%. The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
- Coverage   88.86%   88.80%   -0.07%     
==========================================
  Files          21       21              
  Lines        1707     1715       +8     
  Branches      405      406       +1     
==========================================
+ Hits         1517     1523       +6     
- Misses        190      192       +2     

Impacted Files	Coverage Δ
app-config/src/schema.ts	85.00% <60.00%> (-1.60%)	:arrow_down:
app-config-webpack-plugin/src/loader.ts	95.23% <100.00%> (+0.79%)	:arrow_up:
app-config/src/config.ts	96.47% <100.00%> (+0.04%)	:arrow_up:
app-config/src/index.ts	95.58% <100.00%> (+0.06%)	:arrow_up:

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