WIP: convert to use ES modules, setup unit tests with karma

[web-padawan]

Connected to vaadin/components-team-tasks#424

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This change is 

[limonte]

Any reason not to use yarn.lock? There are more than 2 thousands of dev-deps and the recent accident with event-stream package demonstrated why lock files are important, read more: https://schneid.io/blog/event-stream-vulnerability-explained/

btw npm install results with

found 5 vulnerabilities (1 low, 4 high)

It's not a wise practice to just ignore them.

[web-padawan]

Review feedback addressed. For now moving back to "Design & discussion" as agreed.

The topics left out of the initial scope are:

• visual tests
• test-fixture replacement
• stylelint (possible if we use lit-css and StyledLitElement`)
• prettier (worth trying once Change HTML/JSX formatting to have one attribute/prop per line prettier/prettier#5501 is done)

[moderndeveloperllc]

If you are going to use eslint and lit elements, might want to check out adding eslint-plugin-lit to the babel parser plugins list. has a decent collection of rules to help with LitElement linting.

[web-padawan]

@moderndeveloperllc yes, Vaadin platform 15 will drop IE11 support and in practice that means that next major versions for all our components will not be tested against it anymore.

[web-padawan]

Closing in favor of #207 which contains a rebased and squashed version of this PR.