build(deps): move Pixelmatch from dependencies to devDependencies

[vlakoff]

Summary

A small follow-up to #206.

Moves the Node.js dependency Pixelmatch from dependencies to devDependencies in package.json.

Why

• Pixelmatch is only used in unit tests, not in production code.
• With this change, it still installs on npm install, but is skipped when running npm install --production.
• Improves self-documentation of the project by making it clear that Pixelmatch is a dev-only dependency.

Refs

From this answer on Stack Overflow:

• devDependencies are:

  • also installed on npm install on a directory that contains package.json, unless you pass the --production flag, or if the NODE_ENV=production environment variable is set
  • not installed on npm install "$package" on any other directory, unless you give it the --dev option.
  • are not installed transitively.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.81%. Comparing base (2edcbd5) to head (d141655).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #216   +/-   ##
=========================================
  Coverage     70.81%   70.81%           
  Complexity      994      994           
=========================================
  Files            49       49           
  Lines          3169     3169           
=========================================
  Hits           2244     2244           
  Misses          925      925           

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[vlakoff]

Follow-up PR, fixing an oversight: #219.