feat: Add rules to enforce arrow functions

[AntonioVentilii-DFINITY]

Motivation

To further help with enforcing the arrow functions we add two rules and one plugin. This is one of the most suggested combination I found online:

• prefer-arrow-callback: this will enforce arrow functions when given as callback to other statements;
• func-style: the default settings of this rule forces to avoid using declaration statements for functions, for example function foo() {...}
• prefer-arrow plugin: with the configs set by us, the plugin will generally make notice of the usage of function.

Changes

• Installed plugin prefer-arrow.
• Include rules in index file.
• Fix issues raised.

Tests

The rules raised some new issues on this repo and we fixed them.

[AntonioVentilii-DFINITY]

@peterpeterparker note that the new plugin does not seem to be so well maintained (last version 4 years ago). However there is not much to mantain except bumping dependencies.

There is an alternative with this other plugin that is a fork actually and seems more recent: https://www.npmjs.com/package/eslint-plugin-prefer-arrow-functions

What would you suggest?

[peterpeterparker]

Do we need the plugin? Maybe the rules already cover enough use cases at least to get started?

[AntonioVentilii-DFINITY]

I checked and it would have not raised any issues, without that plugin, in this repo. What I can do is to create a "simpler" rule like the one provided in the new plugin but limited to our usage

[peterpeterparker]

I see. Sounds cool to me if you think it makes sense? Your call.

[AntonioVentilii]

So, update here, @peterpeterparker :

I tried to create our own rule, trimming the one provided by those library. But it is basically an entire copy/paste minus 10/15 lines (over 350 lines total)...

So, I think it is ok to use the library we are currently using (it had more than 1 million download in the last week), and we can always replace it with our own code in the future, IF and WHEN the time arrives.

WDYT?

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