Replace textual guidelines with ESLint rules where possible

[fregante]

Instead of a 590-words styleguide it could be shorter and the rest can be enforced by eslint, where automation is possible, like:

• Use tab-indentation and semicolons.
• Don't prefix the name of interfaces with I; Options, not IOptions: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/interface-name-prefix.md
• Imports, destructuring, and object literals should not have spaces around the identifier; {foo}, not { foo }.
• Use the array shorthand type notation; number[], not Array. https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-array-constructor.md

[sindresorhus]

This has always been the goal. I just needed something immediate as automation will take time.

This is mostly blocked by xojs/xo#373. The intention is for XO to lint ambient files (index.d.ts) by default.

I'm also holding out for typescript-eslint to be more mature. I don't want to have to disable rules in hundreds of modules because of typescript-eslint bugs. It's also missing a lot of needed rules.

[sindresorhus]

If anyone wants to help out. Find things in the style guide that can be automated, check if typescript-eslint has a rule for it, if yes, comment it here, if not, open a rule request on typescript-eslint and comment the issue link here.

[fregante]

I noticed Airbnb actually still has the verbal styleguide in addition to the shareable config. Not something I'd personally read, but perhaps some people do.