ESLint: Explicitly unignore ESLint config

[tfrommen]

Description

ESLint, by default, does not lint the .eslintrc.js file.

This PR fixes that.

How has this been tested?

Run this:

./node_modules/.bin/eslint .eslintrc.js

Without the new entry in the .eslintignore file, ESLint will report that it is ignoring the config file.

With the new entry, all is good (or not, which would mean issues with the contents of the file 😉).

Types of changes

Unignore .eslintrc.js for ESLint.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.