Add prettier to xo config and switch to two-space indentation

[Kreeg]

@XhmikosR for me, conversations in commits are quite inconvenient. Let's communicate here.
Have you seen this changes? Do you agree/disagree with them?
Sorry in advance for this PR.

[XhmikosR]

So, here we have 2 choices:

1. try to stay close to the xo style we are already using
2. go with prettier's defaults and adapt xo

In the first case, not sure what the gain is, though. Just being able to format other files except for js/ts etc?

I don't mind going with the second way, as long as we agree on a common set of options.

[Kreeg]

@XhmikosR let's go with second variant. Then step by step, file by file we will include prettier to all code base.
What line length do you prefer? I'm OK with 120. We may decrease it to 80

[Kreeg]

@XhmikosR should we land it?

And by the way, I think we should change trailingComma to es5 to add commas after each field in object and all elements in array. This is better for code review since it will cause the one-line change instead of two lines (comma + line break + new field / new element )

[XhmikosR]

Please hold on to this. I haven't had a chance to look at it yet.

[XhmikosR]

Is there any way we don't touch the license headers?

[Kreeg]

Yes, https://github.com/hosseinmd/prettier-plugin-jsdoc/#Options
jsdocCapitalizeDescription: false

[XhmikosR]

So far I see the following:

1. I don't like prettier using single quotes in yaml, but I can live with it. If there's a way to use double quotes for yaml/yml files only feel free to push
2. I don't like the license header changes; they feel unnatural :/
3. I don't like the new operator-linebreak option, but I guess I can live with it
4. space-before-function-paren: can't we enforce this to never like we had?
5. Indentation in jsdoc seems wrong now. For example:

   /**
    * Checks if value is a callable function.
    *
    * @param   {any}     value The value to check.
    *
    * @returns {boolean}       Returns true if value is correctly classified, else
    *   false.
    */

6. I definitely don't like prettier's HTML formatting so I have disabled it (feel free to disable it in a better way if there's one)
7. prettier chokes on our templates
8. format check is not enforced on CI; we should probably add it there too if we go with this approach
9. I switched to two-space indentation because it feels more natural, but it's hard to see the diff because it's huge

Overall, I don't mind proceeding with this as long as we sort the above :)

[Kreeg]

@XhmikosR this is a big list. I think in this case we should not use prettier. Otherwise we would struggle to fight against it. Prettier is a zero-config like solution, if there are so much factors with it's integration, then we can say it just does not fit here

[XhmikosR]

Closing for now. I landed the 2-space indentation in #812