As a developer working on the EntitySchema extension, I want to make sure the JS linting rules align with current common practices
**Problem:**
The EntitySchema [linter config](https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/EntitySchema/+/refs/heads/master/.eslintrc.json) is a few years old and not up to date with common technologies/pratices
**Example:**
[This change](https://gerrit.wikimedia.org/r/c/mediawiki/extensions/EntitySchema/+/900644) adds es6 support to the linter config.
Suggested improvements:
- Make `eslintrc.json` an easier to work with, by making it a `.js` file
- Require trailing commas in multiline arrays / objects
In general we want to take a look at more current projects to see what rules are used there (for example [Special:NewLexeme linter config](https://github.com/wmde/new-lexeme-special-page/blob/main/.eslintrc.js))