Implement includes operator

[stavrogyn]

Description

includes is implemented according to the issue

The operator checking if a store (containing either a string or an array) includes a specified value.

Checklist for a new method

• Create a directory for the new method in the src directory in param-case
• Place the source code to src/method-name/index.ts in ESModules export in camelCase named export
• Add tests to src/method-name/method-name.test.ts
• Add fork tests to src/method-name/method-name.fork.test.ts
• Add type tests to test-typings/method-name.ts
  • Use // @ts-expect-error to mark expected type error
  • import { expectType } from 'tsd' to check expected return type
• Add documentation in src/method-name/readme.md
  • Add header Patronum/MethodName
  • Add section with overloads, if have
  • Add Motivation, Formulae, Arguments and Return sections for each overload
  • Add useful examples in Example section for each overload
• Fill frontmatter in src/method-name/readme.md
  • Add title. Make sure it uses camelCase syntax just like the method itself
  • Add slug. Use param-case to write it. In most cases it will be just like title
  • Add desription with one short sentence describing what method useful for
  • Add group. To categorize method on /operators page. Full list of available groups, you can see in documentation/src/content/config.ts

[earthspacon]

a must be either an array or a string, so I would write it like this:

if (typeof a === 'string') {
      return a.includes(b as string);
}

throw new Error('First argument should be a unit of array or string');

I'm not a maintainer, just decided to leave a comment :)