feat: get code coverage from component tests

[bahmutov]

• closes #70
• uses https://github.com/artberri/rollup-plugin-istanbul to instrument the Svelte component files
• reporting using https://github.com/cypress-io/code-coverage

The report looks a little weird, I am not sure if Svelte files are really counting the coverage correctly

[bahmutov]

:tada: This PR is included in version 2.2.0 :tada:

The release is available on:

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

Your semantic-release bot :package::rocket: