es6: prefer String.startsWith() and String.includes()

[NovemLinguae]

String.startsWith() and String.includes() were introduced in ES6 and are great. Very readable.

• I'd like to suggest a rule that looks for String.indexOf(a) !== -1 and String.indexOf(a) === 0 and warns or errors for these, telling the user to change them to String.startsWith() and String.includes()
• Could also auto fix these

Source code from another eslint plugin with these rules here and here (MIT license)

[edg2s]

We can use the already present unicorn plugin for startsWith:

• https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-string-starts-ends-with.md
  although it only will detect a regex, as indexOf could be form an Array, and we can't tell the difference without typescript: prefer-starts-ends-with: Support string.indexOf('') === 0  sindresorhus/eslint-plugin-unicorn#712

[edg2s]

Alternatively the rules could be applied to ES2016 rules only, where Array.indexOf can also be replaced by Array.includes, and so the type wouldn't matter