feat: instrument code coverage by default

[bahmutov]

• closes #338
• sets the plugins and support files to require code coverage
  • cypress-experimental will need an update to point at those files
• add CI job that takes a scaffolded app and adds component tests and runs them
  • first using the included version of cypress-vue-unit-test from cypress-experimental to make sure it works
  • second using this development version of cypress-vue-unit-test built at the start
• it would be nice to first copy the example project into outside fresh folder to avoid node_modules being discovered up the parent chain accidentally, but it can be done in a separate issue

[bahmutov]

:tada: This PR is included in version 3.1.0 :tada:

The release is available on:

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

Your semantic-release bot :package::rocket: