Rewrite UI and add Jest tests

[robmosca]

THIS PR IS STILL A DRAFT. NOT READY FOR REVIEW.

DISCLAIMER
I created this PR on top of #89.
This PR includes all the changes of

• Add Jest tests for api #87
• Add tests and automated publishing the package to NPM #89

It addresses issue #22.

With the accumulation of the three PRs this one is quite big. I would normally break it down in several smaller PRs but I considered including all changes in one, in the hope it would accelerate the process. I would be happy of breaking the PR down in smaller chunks if we can agree on a reviewing plan.

MOTIVATION
The import-map-overrides code is currently not covered by Jest tests. This makes updating the project rather risky, as any change needs to be manually tested to make sure no regressions are introduced and the manual process might be missing potential errors.
Also, the interface looks rather outdated with a spartan styling of components. In addition to that, the code still uses class components, while the practice nowadays is to use functional components, for their simplicity and ease of maintenance.
Finally, the repository does not include any automation for deployment, a thing that has led in the past to problems for the deployed version of other packages in the project.
This PR tries to remediate all these problems.

CHANGES INCLUDED
This PR includes changes from other 2 PRs (see above). We will summarize here the list of changes introduced:

1. Added Jest tests covering the API (see Add Jest tests for api #87)
2. Added publishing automation using the changesets GitHub release action (see Add tests and automated publishing the package to NPM #89)
3. Rewrote all preact components as functional components
4. Refactored the logic for calculating the different types of modules in a context (ImportMapOverridesContext) which allows sharing information to several components in the hierarchy, leading to a more responsive UI where changes can be reflected in several places. For example, the button now can change color to indicate that there are pending changes that need a reload. This also allows better encapsulation and testing of that logic.
5. Added Jest tests covering all UI components. The current test coverage for the project can be seen in this image:
   
6. Revamped the UI:
   • Modernized and reorganized the element's style
   • Added action items in the table of overridden modules, to simplify interaction with them
   • Introduced light/dark themes matching the browser theme
   • Fixed minor inconsistencies. For example, while the UI shows a reload button for modules in nextOverriddenModules it does not do the same for modules in pendingRefreshDefaultModules, even if they also would require a reload to become effective).
   • The trigger button now only appears when the popup is closed. Its color reflects the status of modules: grey if there is no override, yellow if there is an override pending a reload, red if there is any active override.

You can see how the interface looks like in the following video and images:

imo.mp4

[robmosca]

I will close this PR and reopen multiple PRs addressing the change bit by bit.