Allow usage of flat config

[onigoetz]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

As explained in #190, ESLint has a new config system.
This is meant to be the future default but it does not work out of the box.

This PR aims to allow the usage of flat configuration for pioneers while being future proof at the same time.

Breaking Changes

None

Additional Info

The new API is documented here: https://eslint.org/blog/2022/08/new-config-system-part-3/

Changes introduced by this PR:

• new configType option to allow to switch between the legacy and future way of loading configuration
• changed tests to use async/await where possible, to not wait for the timeout to see the errors
• moved all code to load eslint to worker.js (was duplicated in getESLint.js as well)

Each change is in its own commit, you can review them separately.

I decided to pile this PR on top of #229 because it changes the same areas

[alexander-akait]

@ricardogobbosouza Hello, can you look at this?

[ricardogobbosouza]

Thanks @onigoetz
It might be better to base it on the master branch.
I will do this to separate the PRs

[onigoetz]

sure, I'll make a separate one that's based on master

[onigoetz]

Hi @ricardogobbosouza, in the end it was quicker than I expected, I opened #238 with the minimal amount of changes required