Added linter and formatter

[toni-santos]

Closes #20
Add eslint and prettier

[toni-santos]

I have not yet touched on the popup's files prettier and eslint configs, seeing as it would be pretty cumbersome to have 2 (perhaps different) instances of these tools affect the code in different ways. Those files have been however formatted/linted according to the rules introduced in this PR.
Proper testing for this PR is also important so it would be good to have multiple people actively hunting for issues introduced by tweaking so much code.

[thePeras]

Have tested the core features and nothing is broken!

Don't forget to

• Commit a recent formatted version (package.json is unformatted)
• Delete the popup settings
• Update the readme with the new scripts commands

What you think to add this to readme?

.vscode/settings.json
{
    "editor.formatOnSave": true,
    "editor.defaultFormatter": "esbenp.prettier-vscode"
}

So people automatically format on save using the added formatter.

[thePeras]

After fixing the conflicts, ready to go! ✅ 🛫

[thePeras]

To setup the action we can check TTS PR.
An important aspect is to make it running only on ready PRs and ignore Drafs!