feat(eslint-plugin): add prefer-includes rule (fixes #284)

[mysticatea]

Fixes #284.

This PR adds new @typescript-eslint/prefer-includes rule.

This rule suggests using includes method if:

• all of the following conditions satisfied on indexOf method calls:
  1. the call is used to check if the receiver object contains a given value or not.
  2. the receiver object has includes method also.
  3. the indexOf method and includes method have the same parameters.
• all of the following conditions satisfied on test method calls:
  1. the receiver object is a RegExp object.
  2. the content of the RegExp object can be determined to a unique string.

This PR adds two packages as same as #289.

[codecov]

Codecov Report

Merging #294 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
+ Coverage   96.87%   96.95%   +0.08%     
==========================================
  Files          70       71       +1     
  Lines        2625     2694      +69     
  Branches      424      435      +11     
==========================================
+ Hits         2543     2612      +69     
  Misses         49       49              
  Partials       33       33

Impacted Files	Coverage Δ
...ackages/eslint-plugin/src/rules/prefer-includes.ts	100% <100%> (ø)

[mysticatea]

Side note:

Because I have "format on save by ESLint" always, I have sometimes forgotten to run prettier manually.
I wanted to use .vscode/settings.json to enable editor.formatOnSave option.

dissmissed

[bradzacher]

one nit otherwise lgtm

[mysticatea]

Thank you for the review!
I updated this PR.

[JamesHenry]

Thanks a lot! Just need to resolve the conflicts

[mysticatea]

Hi. I updated this PR to resolve conflicts.

[sindresorhus]

This rule was not added to the readme: https://github.com/typescript-eslint/typescript-eslint/tree/master/packages/eslint-plugin#supported-rules