style: add eslint and prettier

[Aakodal]

Litarvan: feel free to change rules you disapprove.

EDIT: this PR is left aside until Litarvan decides to refactor the whole project (not my words, but his).

[JezerM]

The results of the Prettier config should be similar as similar as before, without introducing a lot of changes in a simple PR.

[Aakodal]

The results of the Prettier config should be similar as similar as before, without introducing a lot of changes in a simple PR.

Oh well that didn't change a lot xD
I changed tabWidth to 4 since I committed your suggestion (4 and not 2 because it was 4 before this PR), but it looks like there are still more than 20k lines changed, mostly caused by code inconsistency.

In that case, shouldn't I revert to tabs? Even if final rule is up to Litarvan, I'd recommand them since everyone can configure how they look, and they are literally meant for indentation.

[JezerM]

Well, yeah, at then end it is Litarvan's decision~

[JezerM]

package-lock.json is the most changed file with +20,000 changes; if you don't want to change much at all, I would recommend to set tabWidth: 2 for all json files.

Suggested change

	singleQuote: true
	singleQuote: true
	overrides:
	- files: '*.json'
	options:
	tabWidth: 2