feat: add code coverage

[bahmutov]

• closes Documentation for code coverage? #322
• disable coverage by using an environment variable

{
  "env": {
    "coverage": false
  }
}

• include code coverage plugin and generate coverage reports
• broke i18n test, will skip for now, opened separate update and fix i18n spec #332

based on https://github.com/bahmutov/code-coverage-vue-example

[bahmutov]

seems upgrading the vue-loader is causing problems

[bahmutov]

babel-plugin-istanbul instruments Vue files, but only if there is .nycrc file with

{
  "extension": [".js", ".vue"]
}

Problem is how to pass these options without creating this file (or requiring any other NYC config
options)

just for fun, nyc options from defaults and above file

this.nycConfig {
  cwd: '/Users/gleb/git/cypress-vue-unit-test',
  exclude: [
    'coverage/**',
    'packages/*/test{,s}/**',
    '**/*.d.ts',
    'test{,s}/**',
    'test{,-*}.{js,cjs,mjs,ts}',
    '**/*{.,-}test.{js,cjs,mjs,ts}',
    '**/__tests__/**',
    '**/{ava,nyc}.config.{js,cjs,mjs}',
    '**/jest.config.{js,cjs,mjs,ts}',
    '**/{karma,rollup,webpack}.config.js',
    '**/{babel.config,.eslintrc,.mocharc}.{js,cjs}'
  ],
  excludeNodeModules: true,
  include: [],
  extension: [ '.js', '.vue' ],
  coverageVariable: '__coverage__',
  coverageGlobalScope: 'this',
  coverageGlobalScopeFunc: true,
  ignoreClassMethods: []
}

[bahmutov]

🎉 This PR is included in version 3.0.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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