Add missing lib folder to package.json's files

[planger]

Change-Id: Icdf84ef17c208bf90a77abf3ef71e9b474d4ec1c

What it does

Add the lib folder to the files property of the package.json.
Otherwise the compiled sources aren't included when the package is installed as a dependency.

How to test

Once published, the compiled dependenciesin lib should be published.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[planger]

I'm afraid we missed to include the lib folder (see also https://community.theia-ide.org/t/using-theia-playwright-framework/2339/4=). In our downstream projects, we for now only consumed this package by including the sources. So we didn't catch this issue.

@vince-fugnitto can you please have a look and merge this, if you are ok with this change? Once published, it should then include the compiled sources.

[vince-fugnitto]

LGTM!